The Pactmaker is not a religion in the same sense as the other eight deities, the Pactmaker is a bit of an enigma. He offers six pacts with varying effects, and does not require you to "join" his religion. Instead, the chosen pact will siphon piety away from your current religion to fuel their effect. This allows the Pactmaker to complement other gods and create unique combinations. This enables players to use deities that do not offer any worthwhile boons for their strategy, so long as they are capable of earning piety. The Pactmaker also offers the consensus Pact, granting a massive influx of piety instead of draining it.
You can only ever request a single Pact, and it can never be disabled (though if you run out of piety it will not take you into the negatives) so always choose wisely and ensure you have enough piety income to fuel the pact in question. The Pactmaker will appear sporadically throughout the game, and will only be unlocked once you have unlocked every other altar. He is widely regarded as one of the most powerful deities in the game for his ability to increase the effectiveness of every other deity.
The Pactmaker is the only altar that cannot be desecrated.
Because the Pactmaker can be combined with any other deity, he has by far the broadest application and is almost always worthwhile.
This Pact is very useful for characters who are performing XP catapults to quickly level-up, granting a one-time bonus every time you gain a level. It's extremely powerful for any character who needs to get more mileage out of his full heal on level-up. Naturally, this works even better if you've increased your hit points using boons such as Absolution or Boost Health, but it's not very useful for spellcasters. This can also be very expensive if you select the Pact early.
This boon is quite expensive and has a lesser effect than most of the other boons. If you're just a few notches away from leveling up it can be a great "last minute" pact to eke out a little more XP, but the high piety cost and low effect means it's seldom worthwhile unless it's just to push you over the edge.
A great Pact for Rogues or anyone who needs to get a little bit more HP to survive the boss. The Warrior's Pact can easily turn a large number of popcorn monsters into a healthy layer of extra hit points. However, the piety cost is quite significant and can quickly drain you if you don't have enough piety income to maintain it. Tikki Tooki, Taurog, and Dracul (with blood tithe and blood hunger) make the best choices when using this boon since they will grant piety to offset the cost whenever you kill a monster. Worshippers of Jehora Jeheyu are advised to use his "boost health" boon rather than attempt to use this Pact.
Very similar to the Warrior's Pact, the Body Pact will allow you to accrue resistances. This is both the most powerful and most expensive of the Pactmaker's pacts. You usually need to begin using this pact early, since it only triggers if the monster successfully attacks you. You also need an enormous amount of piety income to maintain it. This boon works best with the Earthmother, Binlor, Taurog, Mystera, and Glowing Guardian; deities that can generate large amounts of piety for low-level characters.
Although the amount of conversion points appear low, they can add up if you convert some low-cost items that are bought from shops. The piety cost can be a very difficult to sustain, and the benefits can be difficult to accrue in large amounts. This pact is usually only worthwhile for characters which literally have nothing better to spend their piety on, and it's almost always preferable to go with other Pacts. For instance, Dwarves will almost always end up with significantly more hit points at a lower piety cost by using the Warrior's Pact and Goblins will get more XP by using the Alchemist's Pact. Unless you're a human or orc desperate for damage or an elf desperate for mana, there's little reason to pick this pact over the other superb options offered here.
Very simple: your piety maximum is reduced to 50 and you get filled up to this lower maximum. For most deities, this isn't a big deal, and if you're having trouble earning piety it's unlikely you'll hit the lower cap. This pact works well if you get stuck in a religion and cannot convert out, or if you desperately need a specific boon.
You can still select a boon after using consensus, but you cannot select consensus after selecting a boon (so you can't just make a pact and still get some free 50 piety during the endgame). Pay careful attention to the order in which you do things, because this particularly pact option is quite fickle.