While he's not a god, Eltab was a demon lord that was imprisoned under Thay (in the Forgotten Realms), and his breaking free was the culmination of the novel The Crimson Gold.

Similarly, Bazim-Gorag was a slaad lord who was also imprisoned in the Realms.

Baldur's Gate: Descent into Avernus notes that the infernal demigod Gargauth is trapped in the Shield of the Hidden Lord (though admittedly, you find him in the city itself rather than Avernus).