- Rheanimum is a substance is in the Generation One continuity family.
Marvel Comics continuity
When the Swarm attacked the combined Autobot-Decepticon forces on Earth, the rheanimum was distributed among the allied Transformers, in hopes that its properties would render them invulnerable to the Swarm. It worked. However, in Jhiaxus' flagship in Earth orbit, Optimus Prime refused the rheanimum offered to him by Starscream; in order for his desperate gamble to work, he needed to be consumed by the Swarm!