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The first game I ever really experienced "gaming addiction" with where I spent a single Saturday playing from 1PM to 1AM was Magestorm.
It was like Doom/DukeNukem style 3d with 2d sprites but you could level up and stuff and there were various classes. It was 3-way realm combat, and was probably the first RvR game as they continued that 3-team concept into future projects eventually leading to DAoC which influenced the design of ESO and various other 3 team games.