Beast Wars: Transformers S03E10 - Master Blaster

During an attack on the Maximal base, Optimus is hit with a burrowing cyberbug that allows Quickstrike to take control of his body. Tarantulas and Quickstrike form an alliance in order to destroy the Ark and Megatron. Tarantulas convinces Megatron to use the spark of the original Megatron to upgrade himself as Optimus did. Once Megatron absorbs his namesake's spark, Tarantulas has Optimus (controlled by Quickstrike) drop Megatron into the lava. Blackarachnia manages to stop Tarantulas, using her past knowledge of his work. After he was thought to have "died", Megatron rises from the lava, having now become a gargantuan red fire-breathing dragon.