When Fungal Mesh Unleashes, if there is at least one prepped spell, any player casts a prepped spell. The damage that spell deals must be dealt to gravehold or any player instead of a minion or the nemesis.
If the players choose to cast Combustion (Cast: Deal 2 damage to a minion. Deal 2 damage to a different minion or nemesis.), the Unleash effect overrides the targeting restrictions on Combustion. Both instances of damage can be dealt to any player or Gravehold.