Rare Ergo is a unique item in Lies of P that can be obtained by defeating main bosses. It can be consumed for a large amount of Ergo or traded with Alidoro the Treasure Hunter for a unique boss weapon or amulet. Here are some things you can do with Rare Ergo:

1. Trade with Alidoro: Alidoro is an NPC that can be found in Area IV, St. Frangelico Cathedral. He will trade Rare Ergo for unique boss weapons or amulets. Each Rare Ergo has a specific weapon and amulet available for trade, so players must decide which one to purchase. It is recommended to hold onto Rare Ergo until you encounter Alidoro to see if there are any interesting items that work with your build.

2. Consume for Ergo: Rare Ergo can be consumed for a large amount of Ergo, which can be used to level up Pinocchio. However, it is important to note that Rare Ergo is a unique item, and players can only get one of each bosss Rare Ergo per playthrough. Therefore, it is recommended to hold onto Rare Ergo until you know what you want to do with it.

In summary, players can trade Rare Ergo with Alidoro for unique boss weapons or amulets or consume it for a large amount of Ergo. It is recommended to hold onto Rare Ergo until you encounter Alidoro to see if there are any interesting items that work with your build.