Missa pro defunctis is a composition for four voices by the late Antoine Brumel, a French Renaissance composer. This work is notable for being the first Requiem setting to include an alternatim polyphonic setting of the Dies Iræ (a setting in which sections of plainchant alternate with sections in polyphony). In addition, it is one of the earliest surviving requiems (only Ockeghem's Missa pro defunctis is earlier).
References
Missa pro defunctis (Brumel) Wikipedia(Text) CC BY-SA