Grave Cairn is very thematic, but suffers from the problem that if a character is leaving play, it probably has low attack. The only in-sphere options that are possibly worth it are the Dwarven Sellsword and the Greenwood Archer. Of course Grave Cairn fits well into a gimmicky deck that tries to sneak attack Gandalf a lot or make frequent use of Reinforcements.
Response: After a character leaves play, add its to another character's until the end of the round.
Legolas, The Two Towers
The Watcher in the Water #54. Leadership.
I think that this is a very powerful card against tough enemies like trolls and dragons, if Gandalf, Gandalf, Saruman, Giant Bear, or Beorn for example are leaving play, something which they tend to do quite a lot.