Echidna is a weird creature that eats bugs and is highly electrosensitive (with apologies to Jacob Stanley) More seriously, Echidna is a Haskell program designed for fuzzing/property-based testing of Ethereum smart contracts. It uses sophisticated grammar-based fuzzing campaigns based on a contract ABI to falsify user-defined predicates or Solidity assertions.
This reference consolidates EVM opcode information from the yellow paper, stack exchange, solidity source, parity source, evm-opcode-gas-costs and Manticore. New issues and contributions are welcome, and are covered by bounties from Trail of Bits.