déchainer

The spelling recommended by the 1990 orthographic reform for déchaîner, dropping the circumflex accent on the i since it creates no ambiguity — both spellings mean exactly the same thing (to unleash) and both are taught in French schools today, though déchaîner with the accent is still far more common in print.

• La foule s'est déchainée à l'annonce du résultat. — The crowd went wild when the result was announced.
• Ses critiques se sont déchainées sur les réseaux sociaux. — His critics went on the attack across social media.
• Le chien s'est déchainé en voyant le facteur. — The dog went crazy when it saw the mailman.

This is simply déchaîner without the circumflex — same conjugation, same meaning, same registers. If a dictionary or spellchecker flags it, that's because it still defaults to the traditional spelling, not because it's wrong.

Often confused with déchaîner.