Ter' Reroi were created by the Forerunners to adapt like how the flood does, but eventually were lost aboard a space transport. Later they were found by the Covenant Seperatists and gladly cooperated with the Covenant for a still unknown reason. They can mold there metal bodies into anything they scan with a special optic sensor but once molding into that form it cannot turn back. As a result, they have literaly thousands of possible forms and are a legendary foe for the UNSC. They Ter' Reroi are not technically alive, but "think" like the Monitors for the Halo rings, and reproduce using a special method of transferring data and inserting it into a Ter' Reroi built by other Ter' Reroi which were all trained to built more of themselves.They have a standard form, but it is not used for much