Variant Rules (Halo Setting)
From D&D Wiki
The following are all optional, in case a DM thinks their players deserve a bigger challenge.
This one is highly suggested. Ranged attack rolls made while prone are +2. You can also crouch, incurring half the benefits and penalties of being prone. While crouching, you can move half your movement speed.
Covenant weapons: how do they work? In order to have Proficiency in a weapon or armor unique to either the UNSC, Covenant, or Prometheans, a player must have created their character with starting equipment of the same type.
Honestly, there’s no lore explanation for this one, but it’s in the games and would make for an awfully fun game experience. The 'only' way to restore health and temporary hit points is to punch the healing out of your enemies. When you make a melee attack against a creature, you gain hit points equal to half of the damage dealt. If you are granted a regenerating amount of temporary hit points, such as from a Covenant Combat Harness, they do not recharge, but can also be restored in the same manner as hit points.
“Sorry I blew up like that!” Every time a grunt is killed, its methane tank detonates, releasing a blast of plasma. Every creature within 10 feet of the creature must succeed a DC 17 Dexterity saving throw or take 2d6 fire damage.
Lore-Accurate Energy Shields
Shields are designed to negate short bursts from high energy or velocity projectiles, not sustained attacks. Melee attacks deal double damage against temporary hit points added by shields.