To reach the land of shadow youre gonna need to beat two pretty tough bosses first up is starscourge radahn in caelid, and you'll want to trigger his festival by either getting to altus plateau or doing some of ranni's questline. Once he's down, your next target is mohg lord of blood inside mohgwyn palace - you can access this either through varre's questline or find that portal in the consecrated snowfield, whatever works for you.

After dealing with mohg, look for this really distinct withered arm sticking out of a cocoon in his boss arena, that's your ticket in. The timing on both these fights can be tricky, especially mohg's blood flame attacks which have this specific delay you need to watch for. Something worth noting is you can't just pick this from a menu, gotta do it the proper way through the game world.

Level-wise, you're looking at 120-150 with at least 50-60 vigor since the damage output in there is no joke. The neat thing is once you're in, there's this separate leveling system with scadutree fragments and revered spirit ashes that helps boost your power beyond your base stats, which really comes in handy considering how tough the new area can get.