Cleon
Legend
Again, I agree on the planar ally/binding tweaks, but I think that should go in the background text.
Like this?
Devoted Guardian??? (Su): A master bone tyrant is mystically bound to an unhallowed altar by the terms of its summoning. Until it completes its term of service or is released from its agreement, it may not move outside of the unhallowed area. If the bone tyrant is forced outside that range, it is instantly teleported back to a designated area adjacent to the altar, as if by a word of recall spell.
I'd like to include a mention of the 40 ft. radius in the description. We also need to include rules for breaking the altar.
As for the name, I'm thinking something like "Unholy Sentinel". My first idea was "Altar Guardian" or "Bound to Altar", but that didn't sound fiendish enough.
So how about:
Unholy Sentinel (Su): A master bone tyrant that agrees to protect an area is mystically bound to an unholy altar. Until it completes its term of service or is released from its agreement, it may not move outside an unhallow effect radiated by the altar (as the spell, so normally 40 ft. radius). If the master is forced outside that range, it is instantly teleported back to a designated area adjacent to the altar, as if by a word of recall spell.
Should the unholy altar be destroyed, the bone tyrant master and all its minions are instantly banished back to the lower planes. A typical altar has hardness X, hit points Y and Break DC Z.