Much like Mass Effect 3, Inquisition's multiplayer occurs in a separate mode from the main story. The single player campaign has your (Human/Elf/Dwarf/Qunari) (Rogue/Warrior/Mage) directing the Inquisition, while the co-op mode has you playing actual agents operating in the field. This time around, there are zero hooks linking multiplayer and single player (unlike ME3). You can go the entirety of your Inquisition career either solely playing the single player or going co-op exclusive and never see anything suggesting you check out the other mode.
As a proud member of the Inquisition, your task is to join up with three friends, run into Elven Ruins/Tevinter Ruins/Orlesian Chateau and murder everything inside. The context for each mission changes very slightly every time you join a new game, with a variety of characters narrating your expedition between checkpoints. The mission: waste everybody who looks like they might want to slit your throat. That's it, really. Just run in and kill everything. There are a few mid-mission objectives that can pop up between checkpoints, such as "protect this guy!" or "save these documents, which are inexplicably on fire!" but the main mission always focuses on beating everything that doesn't look like you to death.
The co-op is, straight up, no foolin', a multiplayer dungeon crawl. There are twelve classes to play with three of them unlocked from the very beginning. Three is a weird number, considering the mode is built for four players to trudge through the dungeons. Each class possesses two unique skill trees to venture down as you increase in levels, and between missions you can equip up to four skills and four different potions to help you slaughter more baddies. The combat is a slightly simplified version of the single player: click on bad guy to attack, hit a number key to do a special ability, use a potion to heal or fling bees at an enemy. It's pretty par for loot-driven adventure games like this and totally competent. And also they let you throw a jar of angry bees at a demon, which is pretty much the best thing ever.
Forgive me for making the comparison about click-heavy, loot based, fantasy games, but it feels like a 3D Diablo. You're running randomly generated dungeons of the same three tile sets over and over again, harvesting gold and picking up the errant piece of gear (which you're not able to view until the dungeon's end). The process itself is enjoyable, and the classes are different enough keep your interest even after maxing out a specific tree. It doesn't hurt that it's also skull-blisteringly difficult.
Seriously. If your team isn't operating at max capacity, some spikey-armed corrupted Templar jerk is bound to turn your mage into a kebab as your tank absently wails on a hapless weakling. The surprising difficulty makes working as a team and communicating a priority, since failure to do so will result in a record-time KO against certain baddies. It's no wonder that voice chat is active by default.