From the Ashe

Should a Phoenix be slain, it explodes in a shower of flaming cinders. If the Winds of Magic are strong, these fiery fragments swiftly coalesce and the Phoenix is reborn.

When a Flamespyre Phoenix loses its last Wound, roll a D6 before removing the model from play:
• On a roll of 1-2, the Phoenix crumbles into cold ashes and is removed from play.
• On a roll of 3-5, the Phoenix explodes into flame. Every enemy unit in base contact with it suffers D6 Strength 3 hits, each with an AP of -1 and the Flaming Attacks special rule.
Once these hits are resolved, this model is removed from play.
• On a roll of 6, the Phoenix (and its rider, should it have one) are briefly consumed in a ball of flames, and are immediately reborn, recovering D3 Wounds.