BassSanctum

BassSanctum adds a custom dimension. A structure generates with an altar in the middle. The item sitting on that altar is whatever you declare in the config — often a boss summon in a pack, but it can be any item.

To enter the dimension, right-click the block you set in config. Built as a pack-author hook rather than a full content mod: you choose the portal block and the altar reward, then send players in to retrieve it.