Flame orb
From DoomWiki.org
Flame Orbs are the small ammo type used to power Heretic's Phoenix Rod. They provide the player with 1 shot (also on skills 1 and 5), and are sometimes dropped by an Ophidian when killed, although these provide 5 shots. Maulotaurs also occasionally drop flame orbs that can power the Phoenix Rod for 10 shots.