I am writing a feat for D&D 5E, one of the minor features is that the feat causes a creature hit by your critical Slashing weapon attack to receive ongoing bleeding damage until some time has passed or they receive healing. This damage has no effect against Constructs, Undead or Elementals.
What damage type would be dealt from bleeding? Would it be slashing damage, as that is what caused it? Or something else?
It doesn't need to have a damage type.
Example from the Stirge Blood drain ability (MM 284):
Blood Drain. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 5 (1d4 + 3) piercing damage, and the stirge attaches to the target. While attached, the stirge doesn't attack. Instead, at the start of each of the stirge's turns, the target loses 5 (1d4 + 3) hit points due to blood loss.
Under this rule, a slashing weapon makes a "slashing wound". Damage from bleeding out of a slashing wound is still slashing damage. This makes sense because a creature with resistance to slashing damage should receive a smaller wound and bleed less. The same logic applies for piercing and bludgeoning damage.
Necrotic is the damage type for attacks that decay or weaken the body. From a flavor standpoint, it may not be an exact match, but it's the closest single damage type of the 13 listed in the books.
The Sword of Wounding (DMG p207) is an example of this:
At the start of each of the wounded creature's turns, it takes 1d4 necrotic damage for each time you've wounded it, and it can then make a DC 15 Constitution saving throw, ending the effect of all such wounds on itself on a success
I feel untyped damage is likely the best for this, resistance is often something that is limited to how hard it is to inflict damage, this doesn't mean that would necessarily bleed less, and with the possibility of vulnerability and depending on how you handled the amount of damage it could be something a bit more abused. Necrotic strikes me more of a magical nature rather than a bleeding wound and since it is specifically blood loss causing it, well, it would technically be bleed damage.
I'd simply go with untyped and undead, construct and elementals are immune but be careful how you work with damage and how further damage is applied. What one could do is make the bleed for X damage and have x be the attribute associated and have the DC be a flat amount.
