Full Scale Offensive is a brand-new game mode introduced in the Death Note pre-season. Unlike the current Great Rift (which uses cumulative damage scoring), FSO uses a score-based system where you form 3 separate teams — one for each boss.
Each season runs for 6 weeks and features 3 past Great Rift bosses. You'll use 9 combatants total (3 per team), and each boss has unique bonus mechanics you can exploit for extra points.