On one of Jan's side quests I ran into two Githyanki, one of which summoned a Mordenkainen's Sword (or something that looked like it). I've summoned plenty of these against enemies and they have been very effective but I have never had to battle one myself! It's immune to physical damage and seems to be immune to magical damge as well. How do you destroy these things?

(In the end I just ran away from it until it unsummoned... which was not very heroic :()

It is not immune to magical damage like magic missile, finger of death and Abi Dalzim's horrid wilting; but it is immune to phisical attacks, fire, cold, electricity and acid damage. Two magic missile or the marvelous death spell makes short work of the mordaiken's sword.

If you do kill it, make sure to search it's body for a Greenstone Amulet! You can also summon your own Swords and pickpocket them for Greenstone Amuets. ^_^