So you probably saw a vid of a guy using Haunting Memento on Kromog with some... interesting results.
I've been experimenting with other uses, and one i've found most interesting so far is this:
You can use Haunting Memento on Brawler's guild bosses. I have successfully made my friend's hogger turn into a tiny skeleton, making my friend say WTF and then die.
To those who dont know, HM is a toy you can get for 90 darkmoon tickets that on 1hr cd places a debuff on target, giving a chance to randomly make some effect - i.e. make them a skeleton, summon friendly spiders, etc.
This is especially useful on "hard concentration" bosses like Hexus, final tier paladin, Nibbleh, etc