Max level Elden Ring characters won't be able to breeze through Shadow of the Erdtree when it arrives in June. The DLC will use a separate leveling system borrowed from Sekiro, according to director Hidetaka Miyazaki.
In an interview with Famitsu (translated by Reddit user theangryfurlong), Miyazaki said in addition to the normal rune-based leveling system, you'll also have a Sekiro-like "attack power" stat unique to the DLC areas. Presumably, raising it will make you do more damage on top of the stats you already have.
