Legendary Forge

The Legendary Forge is the Blacksmith's workshop, where he upgrades found Gear permanently with Cells.

Upgrading and Gear levels
The way upgrading gear works in Dead Cells is by a level mechanic which can be fueled with Cells. There are three levels of gear that can be achieved with the Legendary Forge:

Level + : Makes your gear overall a bit stronger, Cells make it so the percentage of + items found is higher, its maximum can be reached after spending 500 Cells on the Forge.

Level ++ : Can only be accessed after the + level is maxed out. Makes ++ items appear more often. It makes gear much better and adds one stat scroll to it. Requires 3000 Cells to reach its maximum. On 0BC, it tops out at 50%; on 1BC+; it maxes out at 100%.

Level S : Can only be accessed after the ++ level is maxed out. Makes your gear really, really powerful and adds two stat scrolls to it. Requires 10.000 Cells to reach its maximum. It can only be maxed while playing on 4/5BC, 3BC maxes S at 50%, 2BC at 25%.

Access
The forge only appears twice in a run: after the Black Bridge, Insufferable Crypt or the Nest, and after the Clock Room or the Guardian's Haven.

Trivia

 * The Legendary Forge resembles a furnace and an anvil, the latter being a common platform associated with blacksmiths as they use them to polish weapons.