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Onslaught was taken out of the game during the Trials update.
From the 19.0 [https://dungeondefenders.com/2/blog/142955/trials-the-endgame-update-out-now-full-patch-notes Patch notes], "Onslaught is finally taking the rest we’ve been talking about. In the future, we have plans for an Onslaught revamp or for a new system to take its place. It shall be reborn!"