The theme for Season 22 is a 4th Cube slot and Shadow Clones. The 4th slot can be any item power (although not the same as one of the first three) and the clones are powerful copies of your hero that are spawned when you click on a shrine / pylon, last for 60 seconds (or until your hero dies, whichever is first) and are highly effective at killing things, including GR guardians up to GR100+.
More info on the season and the clones…