Feros Duul sells a wider variety of kataals and spells if you're a member of the Arcane magic guild, and that might be the reason why he had the mandrake root. Also, anyone who wants to use deathlord beware: they have the potential of freezing the game solid in which case your only option is to reboot your computer, and that isn't funny (it's caused by bugs in their AI). Some people even reported that the deathlord turns on you after some time, meaning that it stops attacking monsters and instead it will attack YOU. I'd rather use Summon Evil instead.
