This spell is really sweet in that it is not a spectre illusion lich (which while looks cool, wont let you ride a horse - Arch Lich). Demi Lich is a skele illusion and will allow you to med while sitting on a horse.
Also, a note about our lichs and FD. If you are in lich form and Feign Death (the effect), don't sit there and lich yourself to death; and yes you can do that. Click on the lich buff icon and you will return to flesh form and yet still be indifferent to the mob that might have attacked you. I have seen necro's kill themselves with lich because the didnt realize they could click it off while FD.
Another note... if you are begging KEI and are a Necro, don't come group with me. You are sacrificing allot of mana (killing uptime), because you are using KEI rather than Lich. Lich line spells are the "cat's meow" when it comes to mana regen. Just make sure you get hp stuff as well as int and mana gear so that you have the hps to lich yourself down some and still have a fair amount of hps.
The lich line is also a must have in uber guild raiding and sometimes with grouping, just so you have the mana to give to the cleric or chanter when he / she runs low. Trying to run on KEI or better even, rather than lich only means you are also making the group less than what they could be. The cleric or chanter will run lom in a bad pull and you wont have any mana to give them. You will have essentially killed your group just because you didn't want to lich. lol.
One thing you have to learn is using Vexing Mordina or one of the lifetap line of spells in conjunction with this one so that you are at Full health and full mana at all times. It takes some balancing, but once you got it, you can be the energizer bunny and just keep going, and going.. and going.....