The Computation Orb is a Hardmodeaccessory dropped by the Golem. It makes non-magic/summoner weapons deal 25% extra damage in exchange of consuming 10 mana and being affected by Mana Sickness.
Due to all weapons (excluding summoner) being affected by Mana Sickness, this accessory is most effective with slow weapons instead of rapid ones, like the Chain Gun, since you have more time to regenerate mana with slower ones.
This item is a reference to the light novel series The Saga of Tanya the Evil. In it, Operation Orbs are tools that can cause phenomena through a combination of magic and analog calculation.