A specific condition of the siege subtype is that when you cast it, you have to choose an opponent to defend that battle
Each opponent loses 2 life and you gain 2 life
If we compare it to Boneyard Wurm, we can get a better picture of how good it might be: 3 creatures in graveyardBoneyard Wurm (1G for a 3/3)Ghoultree (4G for a 10/10) 4 creatures in graveyardBoneyard Wurm (1G for a 4/4)Ghoultree (3G for a 10/10) 7 creatures in graveyardBoneyard Wurm (1G for a 7/7)Ghoultree (G for a 10/10) If we construct a shell in which we expect to be able to place a lot of creatures in the graveyard, then Ghoultree compares favorably to Boneyard Wurm
The goal is to keep your characters alive while challenging, even if only with 1 or 2 Willpower left
That flavour text could be a bit dirty if you think about it a bit too much
While this already makes for a particularly unique commander dynamic, Kibo also states that whenever it attacks, the defending player must sacrifice an artifact, allowing you to deprive opponents of key resources as you apply pressure in combat