The Bond of Mutual Suffering feature of the Blood Magic Wizard from Tal’dorei Campaign Setting: Reborn says:
At 6th level, when a creature you can see hits you with an attack, you can use your reaction to bind your vitality to the attacker and force them to share your pain. The attacker takes damage equal to the damage you took.
The Player's Handbook, Ch. 9, says:
Different attacks, damaging spells, and other harmful effects deal different types of damage. Damage types have no rules of their own, but other rules, such as damage resistance, rely on the types.
What type of damage does this feature do to the attacker? Is it type-less damage? Is it the same damage type as the triggering attack deals? Is it something else entirely?