Current Altar locations:
Greensteel Base: Meridia
Altar of Invasion: Shroud Part 1
Altar of Subjugation: Shroud Part 3 & Devil's Battlefield
Altar of Devastation: Shroud Part 5
Eldritch Chamber in the Tower of the Twelve has 2 locked doors.
How about something like:
Upon 100 Total Favor with the Twelve along with the Large Ingredients bag they give you a key that opens up the north door which holds an Alter of Invasion.
Upon 250 Total Favor instead of a 'good job' they upgrade your key to open the south door that has an Altar of Devastation in it.
To not take up inventory space/bank space. We could have the key fit into the Ingredients bag. And it will of course be Bound to Character.