Well, if you'd prefer to save the lost merchant, my Font of Sacrifice to the Rescue addon makes the Font of Sacrifice available that way too.

I completely and utterly disagree with this.

The merchant is SO much better then siding with the Assassin Lord it's unreal.

Killing the Assassin Lord gets you his loot off his body and experience and unlocks the best shop in the game.

Shops rarely have anything good but he always has some sort of artefact, and if you're lucky (or unlucky early game) he can sell the blood vial giving you an extra life.

You also get to craft your own randomized Artefacts, some of which rival and even EXCEED the power of per-determined artefact weapons and armor. Sure, it's not a 100% guarantee to get an upgrade, but this being a roguelike means you're not always going to get that specific item anyway.

The Assassin Lord is a pretty damned good body for clearing most T1s and T2s. Now that the level requirement for the quest has been raised significantly, you won't be using him to wipe most T1s, granted, but, alternatively, he's scaled up and you do have more Possessor talents trained and can therefore make better use of him (including multiple bodies.) I've cleared the entire West with just the AL's body used judiciously and clicking Shadowstep (to weaken or kill the strongest opponent in sight) and Shadow Veil a lot (though also judiciously.) So killing him has a significant advantage, especially if you're playing higher difficulties (which, as a Possessor, you should be) and are likely to be getting great loot from drops anyway.

Edit: I do feel compelled to note that I'm just talking main sequence dungeons. You're not going to clear the Elven Ruins or the Ruined Dungeon or the like with the AL unless you're getting serious help from add-ons (which I currently am, just on a lark, by using an add-on that allows Possessor talents to be chosen by an Adventurer, which makes for a seriously OP combination. Don't try to take out the Grand Corrupter with just an ordinary AL possession.)

Holy crap, they made siding with the assassin lord actually good 0_0

I'm glad I found this little spoiler I would have never found this myself

Asslord is great to side with. Getting access to the font lets you spend some gold to reroll your RNG on items and pick from a list of new results. This results, generally, in a very reliable (for a roguelike) way to upgrade. You also know when it's coming and you can plan around it. But the items you get aren't the best items you can get.

The merchant is paying for new gear of a guaranteed quality. It also rolls extra egos. Your odds of getting something useful are not so great. Your odds of getting something absolutely spectacular do exist though. 100% crit chance from 1 weapon? Absolutely in the realm of possibilities. Not likely, but still very cool.

I go with asslord the vast majority of the time, but sometimes merchant can be nice to mix it up. Also, some classes can't side with asslord at all, so it's nice that the merchant still isn't a horrible pick. It's just not as reliable.

This is true for people who have Forbidden Cults, but if you don't have that, you can't get the reroll benefit from Assassin Lord, which keeps the merchant's side better. It's an interesting bifurcation of the player base experience.

Anything that is adjusted by the Ego Rerolls cannot be sent to the Vault, so if you play using the vault often, you likely will find the merchant to be better over multiple runs. If you don't to Vault runs, then the Assassin's ego reroll is definitely going to be more consistently better.