An essential Quake mod. Before Arcane Dimensions came out it was pretty much the go-to mapper's mod for the game. Like AD, Quoth adds a lot of content for mappers to put in their levels. Unlike AD, the amount of new content is not astronomically large. While recent iterations of AD have an almost excessive amount of content, some of which was developed for basically one custom texture theme employed by one of the mod's huge centerpiece levels, Quoth's additions are specifically curated to be both internally coherent and consistent with vanilla Quake's aesthetic universe.
There are a lot of additions, from the aesthetic to the mechanical, but for most people the star of the mod is going to be the expanded bestiary. I can't say I'm in love with every addition, but overall it's one of the best out there. It works by filling in what were arguably gaps in the vanilla bestiary, like the extremely limited base roster and the "death guard" of knights which get new comrades.
Quoth's more otherworldly beasts include some of the strongest and weakest in the line-up. I'll just highlight a couple to show what I mean. On the weak end, you have the vorelings, which are kind of like fiends that have been neutered to the status of the headcrab, or the flying polyp with its ability to turn invisible. Both of these tend to be nuisances more than anything else, dealing damage which is both small and annoyingly difficult to avoid.
Then there's the gug. It's one of those monsters that can strike you as outright bullshit when you first encounter one, but once you learn its behavior it becomes a master class in downright evil (but fair!) enemy design. It has a devastating melee attack, so you can't get to close. You can't just flee into sniping range either, or it will launch an undodgeable, unblockable earthquake attack. The only way to deal with it without taking massive damage is to keep within range of its other attack, a nasty ball of green acid that splits apart in a V pattern in midair. The upshot is the same kind of delicate footwork that characterizes the shambler dance without being a mere reskin, more harrowing if anything, and the kind of design that only comes with a deep understanding of what makes Quake great in the first place.
Unlike most mods, Quoth doesn't come with a whole lot of actual levels out of the box, but it does include remakes of E1M1 and E1M2 that are well worth playing. Then there are unincluded semi-official levels by mod authors Kell and necros, including an E1M5 remake called Gloomier Keep that would fit nicely within a hypothetical Quothified Dimension of the Doomed. Beyond that, pretty much anything by than, mfx, or RickyT23 is worth checking out, along with Fort Ratsack, Metal Monstrosity, Shifting Planes, Func Map Jam 9... If you want a prime example of the social genius of Quake's modding scene, look to Quoth.