The description of the warlock's Tomb of Levistus eldritch invocation says (Xanathar's Guide to Everything, p. 57; emphasis mine):
As a reaction when you take damage, you can entomb yourself in ice,
which melts away at the end of your next turn. You gain 10 temporary
hit points per warlock level, which takes as much of the triggering
damage as possible. Immediately after you take the damage, you gain
vulnerability to fire damage, your speed is reduced to 0, and you are
incapacitated. These effects, including any remaining temporary hit
points, all end when the ice melts.
The tiefling's Infernal Legacy racial trait grants fire resistance.
For a tiefling warlock with the Tomb of Levistus invocation, how would this eldritch invocation (specific) interact with this racial feat (general)?
- Do they cancel?
- Or does the magic of the Tomb of Levistus invocation (specific)
overpower the Infernal Legacy trait (general) completely?
Which applies here? General vs. specific rules, or resistance vs. vulnerability rules?
Vulnerability and Resistance are not mutually exclusive so there is no general vs specific
The general/specific rule only applies when two rules couldn't occur simultaneously such as two things changing your Strength score, or one thing making you prone and the other explicitly making you not prone. However, with vulnerability and resistance, there is no such clause or requirement and in fact there exists the following quote:
The fact that they apply in an order shows that a creature can be affected by both at once. What this means is that odd damage values will lower by one and even damage values will remain the same. Again, this only occurs because nothing in the rules actually prevents you from having both resistance and vulnerability simultaneously and so the specific/general rule never applies.