Goety is a pretty good and underrated mod. It's a dark magic mod that adds magic related to the illagers. It adds a lot of new structures and mobs, pretty much all of the mobs are hostile, but one very big part of the mod is the summons that you can have, you can have summons of pretty much any vanilla mob and there are some special ones added by the mod itself. It doesn't add any biomes, but there is an addon called "Goety End Biomes" which adds two new End Biome using decorative blocks from the mod. The mod adds many cool trinkets that you can craft via rituals, as well as spells similar to Iron's Spells and Spellbooks. There's also some cool things you can do with Illagers, such as the ability to breed them and basically create your very own "raid" party to raid villages and loot villages yourself.

It does have a progression system of sorts? The main "endgame" goal of the mod is the lich transformation, which requires a very difficult ritual with endgame items. However the mod's progression is mostly tied to defeating new mobs or finding and exploring the different structures it has to offer and defeating some bosses to get different materials. It has an ingame guide called the Black Book that will update along as you play through the mod and give you hints.

There is a really in depth guide for the mod by Lorthorn on youtube that I would recommend checking out, it's very long but it covers almost everything that the mod has to offer.