Ultimate units are similar to Super units, but they can only be summoned via leader-specific Tier 5 Powers. This means that although Ultimate units cost fewer resources than Super units, they could be considered much greater investments.
Like Super units, Ultimate units offer no unit-specific research, and they receive no benefits from global upgrades.
Currently, each faction has only one leader with access to an Ultimate unit. For the UNSC, Anders can invest in the Retriever Sentinel Leader Power; for the Banished, Decimus can invest in the Decimus Drop Leader Power.