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At this point of the game you can also make your way to "Undersigil", accessible via the Clerk's Ward. Down there are some average treasures... and a bunch of continually respawning demons. After you return from the outer planes there is also a 5% chance that a Greater Glabrezu will spawn, which has a chance of carrying one of five really beastly items... but as this amounts to tons of tedious, nihilistic grinding in a game with such a minimal combat focus, I really can't recommend doing it under any circumstances.
If you also can't be bothered with all that grinding but still want the benefits of it, here's the console command for easy experience: