Forgotten Gods is the second Grim Dawn expansion, being mostly notable for adding a number of new Devotion options along with a new class option. The Oathkeeper has a very Paladin-like feel to it and specializes in shields, fire (or alternately acid) damage, and adding retaliation damage to attack skills.
So far I’ve played through Vanilla+Malmouth with 4 Oathkeeper blends (and through FG with 1):
A Sentinel was the first, handily replacing my earlier concept of a retaliation build. Aegis of Menhir and Righteous Fervor are great and the Guardians are fantastic. Meanwhile, Ascension and Presence of Virtue (for this build anyway) offer a wonderful mix of offensive and defensive support. It’s practically a pure Oathkeeper, though the healing and added retaliation from Blood of Dreeg is not to be underestimated. At the moment (post-Malmouth, pre-FG) they look like this; note that I have to reset the Devotions because Dryad wasn’t as useful as I thought it would be.
With storage space getting a bit tight at this point (1 extra tab isn’t really enough for an expansions’ worth of content) it seemed a good time to finally get rid of all the poison-focused gear I’d been collecting for some time now. So I decided to make a Venomblade Dervish (in contrast to the above, this is practically a pure Nightblade). While they certainly kill things quickly enough, they’re remarkably squishy and I think I’ll have to alter the Devotion setup to include Behemoth and maybe Chariot for some more health restoration; the Ancient Grove boss killed it more or less instantly with that multi-eruption thing it does.
Becoming somewhat focused on the Corruption of Gargabol pistol while scrolling through the item database the next character ended up being a gunslinger Oppressor. I had forgotten how effective dual pistols were at efficiently killing things and anything that survives the barrage gets a Bone Harvest to the face. The build was a lot of fun before the recent patch that changed the Terrify effect to Confusion, and will probably be even better now.
Which brings us to the fourth character, a Cyclone Archon, the only one I’ve so far taken through the FG storyline content. Vire’s Might and Eye of Reckoning are both pretty great and synergize well; rush into the middle of a hoard, pop Ascension if it’s a hero/boss, and just start spinning around. It’s such an effective tactic at melting enemies that I’ve often forgotten I even have Wind Spirit. Briarthorn on the other hand is more of a mixed bag. Against most stuff it serves as a perfectly fine damage boost, but against things with AOE death effects (Malmouth’s aetherials for instance) it just dies constantly and the points may be better off in Presence of Virtue.
And what’s that storyline content like? Well, the first half is much like Act 2 visually with an assortment of mostly Beast-faction enemies; dinosaurs, grobles, basilisks, beetles, scorpion-men, gryphons (fat harpies). There are also a few undead (basically just larger zombies) and some ‘Eldrich Spirits’ (repurposed wendigo spirits), which serve as the new enemy faction. The second half in contrast is much more unique visually (lava-flavored interior ruins aside), being mostly populated by Eldrich-faction creatures (poison-flavored Bysmiel mongrels, reskinned ryloks, stuff that looks like the animats from Pillars of Eternity) including re-designed Kymon enemies who prefer fire magic and crossbows to their earlier rifle/lightning setup.
Though it forces you to side with one of the three new factions, doing so does not make the others hostile (all the choice does is change which series of quests you get offered). So there’s no reason not to side with a different faction in each difficulty level (for bounties stick to the Sister Crimson and Brother Segarius ones). You can do the Shattered Realm content immediately or wait until after everything else is finished, and I suggest the latter to make things as smooth as possible. Even then however it’s not really worth going past the 5th Shard with any builds that can’t quickly dish out a ton of damage (the Archon build above at level 72 cleared the 1st-25th Shards without notable issue… although the levels with floor hazards were a pain).
So is the expansion worth buying? Depends on how much you like the Oathkeeper, want/need the new Devotion options, and if you’re interested in the new class of movement abilities (which, granted, fill a much needed mobility hole). If you just want more storyline areas to explore though it’ll probably end up quite disappointing (particularly when compared to Malmouth).