Avellone joins Black Isle Studios, a division of Interplay Entertainment, a developer known for its acclaimed RPGs. This marks the beginning of his professional career in the video game industry.

Avellone serves as a designer on "Fallout 2", a post-apocalyptic RPG that receives critical acclaim. His work helps establish the game's dark humor, moral ambiguity, and memorable characters.

Avellone is the lead designer on "Planescape: Torment", a groundbreaking RPG known for its philosophical themes, complex narrative, and character-driven story. This game becomes a cult classic and is widely regarded as one of the best RPGs ever made. He was the lead designer and writer of the main story arc and of the central companion characters such as Morte, Annah, and Nordom.

Avellone contributes to "Icewind Dale", a party-based RPG set in the Forgotten Realms. His work on the game helps enrich the game world with various quests, story events, and memorable characters.

Avellone contributes to "Icewind Dale II", further expanding the game world and creating new storylines and characters.

Interplay Entertainment, the parent company of Black Isle Studios, faces financial difficulties and ultimately closes the studio. This marks the end of Avellone's tenure at Black Isle.