Irvin, Bernov's pet

Flying, deathtouch, trample
Mount (another creature can 'equip' this one and fuse with this one, their power and toughness combine and they share their abilities and card traits. if a mounted creature gets destroyed, only destroy the mount.)
Irvin, Bernov's pet can only be mounted by Bernov.
When Irvin gets destroyed, if it was mounted, all players sacrifice their creatures. then exile all cards in their graveyards.