King of Scar is a very fun custom game that has recently taken off to some degree of popularity. It is a game that involves skill in 1v1 and tactic on multiple levels. For those of you interested please watch this video of how to, along with game play. add me in game, MrHighways and play some with me. you'll love it.
http://www.youtube.com/watch?v=KTTtxMmOw30&feature=plcp

Question: are champions like Karthus allowed to use their ultimate if they are in the field of battle?

if they are in the field of battle. yes sir.

The problem with KoS is that it turns into a cluster**** towards the end because someone doesn't like losing and ruins it for the other 9.

I've yet to be in a successfully completed KoS game.