Cursed Monster (5e Template)
From D&D Wiki
These creatures live in a realm parallel to ours, causing strange effects when they are summoned over. Many would-be evil overlords conjure and bind these creatures as minions, especially necromancers. This template can be used to prove your villain is a force to be reckoned with, spice up routine encounters, or give your players a reason to prepare Remove Curse.
Any creature other than an Undead or Construct. Keep in mind that higher CR monsters are more difficult to summon, and would not be brought over in large numbers.
Appearance The creature's skin takes on a dark purple hue, and occasionally you can see purple smoke rising off of them and then dissipating.
Other-dimension-ly Any effect that would send a target to their home plane sends this creature to what is generally called the Cursed Realm. It is not able to enter this realm by any known means, and nothing is known about it. When a creature with this template applied is reduced to zero hit points, it dissipates in a cloud of purple smoke and returns to its home realm.
Abilities. The creature's charisma score increases by 3, and constitution score by 2.
Hit Dice The creature gains hit dice equal to twice its charisma mod(minimum one).
Damage Resistances. The creature becomes resistant to necrotic damage.
Languages The creature retains all languages it knows, and learns its summoner's native language, if it doesn't know it already. However, it loses the ability to speak, and gains the ability to communicate telepathically with other Cursed monsters. All attempts to communicate with the creature automatically fail.
Alignment The creature's alignment shifts to represent their summoner's in terms of morality. For example, a chaotic neutral Kenku summoned by a lawful evil necromancer would become a chaotic evil Cursed Kenku.
Cursed Strike When the creature hits a target with a weapon attack(including natural weapons, breath weapons, and unarmed strikes), it may choose to make the attack deal necrotic damage instead of it's normal damage type and cast Bestow Curse on the target as a third level spell without expending a spell slot. It may do this even without having any spellcasting ability. The save DC is equal to 8 + the creature's charisma score + the creature's proficiency bonus (minimum of 10). Bestow Curse cast in this way does not require concentration by the creature. A creature cannot be cursed again by this feature until their previous curse is ended. For lower CR monsters, you may want to give this Recharge(5-6).
Hit points increase according to the additional hit dice and increased constitution score. Depending on which effect of Bestow Curse you favor, they will either have an effective boost to damage output or AC. Remember that already charismatic creatures will have a much greater power boost.