Half-a-Heart Warrior (5e Optimized Character Build)
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Standing alone at the mouth of the cave, a battle-scarred warrior glares at his quarry: an adult red dragon. Engaging in battle, the two fight for minutes on end, trading blows and expertly dodging each other's attacks. Weakened from the dragon's sharp claws and powerful jaws, singed from multiple blasts of fire, the warrior falls to one knee - as the dragon expected. "How foolish", it thinks to itself, also wounded from the fight but still in good fighting form, "for this one to come alone." It unceremoniously blasts the nigh-dead combatant with one final cone of bright flames... except the warrior stands from the flames, somehow unharmed. The dragon tries again, and again, and again, only to be foiled by the warrior's inexplicable ability to not be harmed. It eventually falls to it's wounds, the warrior triumphant at long last.
This is the essence of the build: not to have the most hit points, nor the highest AC, but to resist being reduced to 0 hit points for as long as possible. There are two versions: one reliant on no characters but your own, and one reliant on having as many allies as possible to provide buffs for you. The latter is more powerful, but requires the assistance of your party of specific characters to exclusively cater to your desires. Regardless of the build, the most important features include "if you would drop to 0 hit points, you drop to 1 hit point instead" or some variation of the phrase.
Somewhat underwhelming, the entire point of this build is to have the capacity to absorb a ridiculous amount of damage by preventing most or even all of it by simply choosing to drop to 1 hit point instead, so long as the damage wasn't so great that you would die instantly. Such large quantities of damage are unlikely anyways, and if you're fighting a creature that can drop you to 0 from full in one hit, you and your party are doomed anyways.
The only two important scores for this build are Constitution and Charisma, both of which are required to give the build the highest Constitution saving throw modifier possible. Assuming standard array, you'll want your 15 (+1 for half-orc) in CON and your 14 in CHA.
Race - Half-Orc
As one of the first choices to make for a character, choosing to be a half-orc gives you Relentless Endurance, which is the very first "resist death" effect this build receives. It can only be used once per day, though, so it's not very powerful. On the bright side, it happens without any conditions, and is almost impossible to prevent.
Barbarian Levels - 12
The essential feature to this build comes from the barbarian's 11th-level feature, Relentless Rage. So long as you are in a rage, you can make Constitution saving throws against damage that would normally defeat you, instead reducing you to 1 hit point each time you succeed. With a high enough modifier, it's impossible to fail the saving throw the first few times, even if you roll a 1 (since rolling a 1 on anything other than an attack roll isn't an automatic failure of the roll). This is why you want a high CON saving throw modifier. Taking barbarian as your first level gives you proficiency in these saves, and choosing to maximize your CON with your 4th and 8th-level ASIs will improve these saves further. Your 12th-level ASI will be used to increase your CHA, which becomes important with the following paladin levels.
Paladin Levels - 8
Although barbarian's 20th-level feature is really good for CON saves, it's not the best way to help prevent death from overcoming you using Relentless Rage. What we're looking for here is the paladin's 6th-level Aura of Protection feature, which adds your CHA modifier to all of your saving throws (and anyone you want who's within 10 feet of you) while you're conscious - which you'll hopefully never not be during combat. In addition, your 4th and 8th levels of paladin allows you to maximize your CHA to further increase your CON save modifier.
In total, your modifier for CON saving throws will be +5 (20 CON) +6 (proficiency) +5 (CHA) = +16. This is high enough to allow you to automatically succeed your first 2 Relentless Rage rolls, since the DC only increases by 5 each time and starts at 10. You have an 85% chance of succeeding your third roll, and it becomes 25% less likely to succeed again each time you do. The first time you fail this roll, you use your Relentless Endurance from being a half-orc to give yourself one final chance of succeeding the roll. Now, if you're completely alone, you're only likely to consistently resist dropping to 0 hit points four times each battle, sometimes five. A 6th success is incredibly unlikely, and a 7th is incredibly rare (only possible on a roll of 19 or 20 by the time you're making the Constitution save) in comparison.
If you get to have short rests between battles (from this point, we'll assume you get two) you can resist death more times each day - at least a total of 10 times each day, with a high chance of 13 or 14. More than that is pushing it, but if you have perfect luck, you can resist death a grand total of 19 times each day. By yourself. This is on top of having a regular pool of hit points and your AC.
Clever casters can neutralize this entire build with choice spells. If disintegrate happens to be the spell that drops you to 0, you'll still die. If sleep is cast upon you at any moment after you use your features to not die, you will still fall asleep. Same goes for color spray. Both power word stun and power word kill will still affect you normally due to your low remaining hit points. Any other effects that also specify "if this damage reduces the target to 0 hit points" technically aren't prevented by this build either. Not as they were written in the original core books, at least. Many of these effects, like the spell disintegrate itself, have been corrected so that Relentless Endurance, Relentless Rage, and similar features will still prevent you from getting killed in SCAG or reprints, but others like sleep and power word kill still get around your features.
In addition, nothing here will protect you from various conditions. This build only protects against regular damage.
Less underwhelming is the ability to resist death a multitude of times with the assistance of a few loving allies willing to keep you alive indefinitely.
Race and Ability Scores
Remain the same as the previous version for being alone.
Barbarian Levels - 20
Simplistic, but it works. You don't need the high Charisma anymore, only a boatload of hit points and a high Constitution. You'll want to maximize your CON just like before, but this time you get to bring it to 24 with the 20th-level feature. Since you only need 2 ASIs to get there, you have room for three feats to help prevent extremely massive quantities of damage from killing you outright, or at least keeping you alive in every fight for as long as possible.
The Three Feats
Tough. For those beautiful +40 hit points.
Durable. Depending on how your DM interprets the wording of this feat, it is either good for this build or exemplary for this build (and any high-CON character, really). Since we like big numbers, we're going to assume by "roll" it means the roll of the die, thereby allowing you to regain a minimum of 7x2+7 = 21 hit points per hit die you expend. This is higher than the normal maximum of 19. Without the feat, you're more likely to regain ~13.5 hit points with each hit die you expend, and since we're assuming you'll be getting short rests each day, you'll have opportunities to replenish your hit point pool before needing to use your Relentless Rage feature. This feat makes it easier to regain those hit points, and removes the randomness of die rolls since a d12 cannot roll higher than a 14.
Lucky. If you weren't expecting to see this feat, then you should have been. 3 times per day it can be used to reroll your CON saves for even more chances at not dropping to 0 hit points. It may not seem like much, but it'll help with both your CON saves and, if you are ever defeated, with your death saves.
As an alternative to taking feats, you increase your choice of Strength of Dexterity for your levels, the former for increased accuracy and damage with your attacks and the latter for increased AC to get hit less often and therefore have to use Relentless Rage less often.
Without your helpers, you're just a standard barbarian. We're going to assume those working to help keep you alive are also 20th level, and you are part of a standard party of 4. This gives three other characters to keep you alive, the first of which will be a cleric for the casting of death ward, a new effect to prevent you from dying. They will also be able to cast bless on you, giving +1d4 to all of your CON saves as long as the cleric retains concentration. Having a paladin friend for that sweet Aura of Protection at 30-foot range gives you that extra +5 without multiclassing. A bard ally can continually give you inspiration dice for +1d12 when you start rolling low on your high-DC saves for Relentless Endurance. The bard's foresight spell gives you advantage on those CON saves (among its many, many other benefits) and healing capabilities offered by all three can keep you coming back at just a few hit points almost endlessly, even if you eventually succumb to repeated attacks (or a single 6th-level casting of magic missile).
With all the assistance and assuming extreme luck in rolls from the bonus dice, we get: the +16 from solo, +12 (bard's d12 inspiration die) +4 (1d4 from bless) for a maximum possible result of +32 to your CON saves. The minimum is +18, and the average is +25, which in most situations is enough to help you succeed on the first five Relentless Rage uses consistently. You can use your inspirations and luck points after you see the first die roll, too, allowing you to choose if you even want to try for a success in case you've already used Relentless Rage too many times and the DC is too high. In addition, you can use those two benefits in either order.
Ultimately, considering only the maximum modifier, this character with these allies can resist dropping to 0 hit points 9 times with Relentless Rage and once each for death ward and Relentless Endurance for 11 total in a day. Assuming your cleric and/or paladin retain 4th-level or higher spell slots after the first combat, one of them can recast death ward without accounting for short rests. With those two allotted short rests, you and your allies can regain use of precious features to continue your unending rampage. Those 5 bardic inspirations you'll likely have to use to not fall unconscious so many times will replenish and the DC for your save will return to 10 for yet another possible 9 uses of Relentless Rage and one or two more death ward uses with each rest, or 18+2/4 after two short rests for a total of 33 times you didn't drop to 0 hit points, more if you . By this level, most creatures are dealing a couple dozen damage each hit when they aren't sending hundreds of AoE damage your way, so if you are already at 1 hit point when you make use of these various effects, each one is effectively preventing all of that damage.
And so you have become incredibly durable without actually needing any real durability. Just so long as certain effects don't incapacitate or outright kill you, or so long as you don't take hundreds of damage in a single blow, you'll survive just about any battle with the help of a few friends!
Essentially none. In truth, mostly the same as before, but now you have allies to cure those pesky conditions you'll sometimes be afflicted with. All of those same allies are capable of bringing the dead back life as well, so if you are ever subjected to one of the few things that can directly bypass this build and kill you anyways, any one of them has the means to bring you back.