Have felstring Harp equiped, play Seadevil Stinger then Mutanus. You still take damage. Felstring harp should remove the damage and heal the hero 2 but you take the full 8 damage
In regular Hearthstone, paying health is not considered taking damage
You can’t block it, prevent it, or trigger damage synergies by paying health
Seadevil Stinger does not deal damage, what are you talking about?
It makes your next murloc cost health instead of mana, which he expected (wrongly) would be treated as damage.