In a cross-chain task, m active, untrustworthy parties attempt to trade assets using n passive but trustworthy smart contracts residing on multiple blockchains. The decentralized finance industry has developed a variety of tasks, including asset swaps, auctions, futures, etc.. In this paper, we show that there is a particular task, called cross-chain consensus, that is universal in the sense that any protocol that solves cross-chain consensus can be adapted to solve any well-formed cross-chain task. Like classical consensus, cross-chain consensus provides the basis for universal constructions and impossibility results. Nevertheless, cross-chain consensus differs structurally from its classical counterpart by splitting participants into two distinct roles: active, but untrustworthy (Byzantine) parties provide inputs, while passive but trustworthy smart contracts decide outputs.