They are indeed immortal, though council membership waxes and wanes due to the fickle nature of the guild itself.
MaRo mentioned it during Gencon or some similar event when they were unveiling returning to Ravnica. I can't seem to find the article or link of the actual comment but he basically said that while it is the same ghost council, the actual spirits within the group have changed somewhat.
I also haven't played with or against Obzedat in a limited format before, but I hear it's pretty much unbeatable? Just wondering if it might be too strong in a cube with no unconditional removal spells?
Also:
Bounty of the HuntBlessings of NatureIncremental Growth
I'm leaning more towards Bounty though... till I realized the counters are only temporary. Now perhaps Blessings seems better?
and
Algae GharialLumberknot
The fairer or better card? Slapping a
Gift of Orzhova on that tree is no joke!