morior

Morior, mori, mortuus sum is a deponent of the third conjugation -io type: passive in form, active in meaning, so mortuus est is "he died", never "he was died". The present infinitive is the short mori, not the moriri you would predict, and the future participle inserts a syllable to give moriturus. That participle is the one everybody has met, in the greeting the gladiators never actually used: "morituri te salutant".

Two related forms trip people. Mortuus is the perfect participle doing duty as an ordinary adjective, "dead", and mors, mortis is the noun. The verb itself has no perfect active in sight, because deponents have none; where you want "he had died", the pluperfect is mortuus erat.