I’m an old-school fantasy RPG gamer, and I’m rather appalled at the way D&D 5e has proliferated “dark-vision”. Virtually every major ancestry has it except humans. The old game had “infravision” for underground dwellers and “ultravision” for nocturnal creatures. I say ditch it all for “darksight”.

Darksight is for deep underground dwellers only: dwarves and goblins and such. Gnomes, halflings, and others may sleep in holes in the ground, but they aren’t adapted to full life in darkness. They and elves, orcs, and other surface dwellers who are nocturnal or otherwise more active at night merely have superior low-light vision that rarely makes a difference in play.

After 1 round adjusting to darkness, you can see heat in grayscale at 60 feet. You detect objects as well as reptiles, undead, and constructs only within 5 feet, via your body heat, since they have none. You can’t read writing.

Darksight, on the other hand, allows you to see in complete darkness. A living body gives off enough heat to see as by candlelight but not in color. Each person added to a room makes it slightly brighter. But since it is heat you’re seeing by, you cannot read ink-written materials or see the contents of paintings and such. For this reason, dwarves tend to carve runes and figures into the walls of their underground halls rather than paint them or decorate with elaborate tapestries.

Moreover, any vision checks, such as searches, should have disadvantage. Heat-vision is great for maneuvering thru space in darkness, but light still has big advantages. Also, heat-vision is negated when you are in the radius of an actual bright light source, such as a torch.

Undead creatures, of course, do not give off heat (not even from decay, as they magically don’t decay; zombies are an exception, since they’re actually animated corpses). Incorporeal undead are therefore invisible to heat-vision outside the “candlelight” of living bodies. Corporeal undead–mummies, wights, vampires–are, like reptiles, the same temperature as their surroundings and can’t be seen any better than furniture and such.

Undead themselves can see in the dark, of course, as they have magical vision, since they often don’t have working eyes. This might be the same as “true-sight” (or whatever other magic you decide allows one to see thru illusions, invisibility, and such, as well as darkness).