Cumulus is a Polkadot Substrate development framework extension that allows Substrate-built runtimes to communicate with Polkadot parachains via Polkadot’s Relay Chain. The Cumulus consensus engine, which is used to run a Polkadot node, is a critical component of Cumulus. This consensus engine is internally used to dictate to the node and synchronization algorithms which chain to follow, finalize, and treat as legitimate.