Updating the Bone Tyrant Working Draft.I think the Unhallowed Aura and Unholy Sentinel revisions will work.
Hold your horses.But I don't like using summon monster for the summoning. The duration is way too short. I think calling honestly makes more sense for the Master anyway, since there doesn't seem to be anything in the original monster about the Masters automatically getting banished back to the Nine Hells.?
The current draft of the Unholy Sentinel ability has a duration built into it - the duration of the unhallow effect. So we don't need to worry about how long the duration of the spell that summoned/called the bone tyrant is, since it's superceded by the Master's "Become a Sentinel" ability.
So it doesn't really matter how the bone tyrant is brought to the Material Plane, so long as it's there to agree to act as a sentinel and receive its payment.
The negotiation of terms & conditions could be sorted out in advance via some means of interplanar communication, or the use of multiple conjuration/summoning - e.g. summon a bone tyrant minion and give it a scroll with an Unholy Sentinel proposal to pass to its master, then summon it again to get a reply.
I'd rather not invent a new couple of spells for the process, since most of the technical "fiddly bits" are already covered by the Unholy Sentinel power.So I suggest some kind of planar ally for the Master, dropping that bit from Unholy Sentinel, and maybe an additional new summoning spell for the Minions. Maybe the Master automatically summons the Minions when the planar ally negotiations are concluded?
Although I suppose we should consider whether the Master can replace missing minions that have been destroyed by combat or accident, either by summoning them itself or by having a third party do it for them.