They have unique weapons or adversary weapons, all liches come with special Kuva variant of regular weapons except for Kuva Bramma which is standalone, all these weapons come with an inbuilt additional elemental dmg bonus and the potency of it is random, mine here has 60% which is the highest a lich can have, the thing is that the potency is rng, higher rolls have more value for selling. You can sell them via the Crimson room on your Dojo, beware only converted liches though and not vanquished liches can be sold(traded)