It means that your Zombies cost 1 generic mana less to cast. As Geralf's Messenger costs BBB to cast, the reductions does nothing for it (unless there is something like Sphere of Resistance out). Your example with Death Baron is correct, as it has a generic mana in its mana cost which CAN be reduced by Undead Warchief.
August 17, 2016 2:13 p.m.
Abilities like that of Undead Warchief only reduce the generic mana of a zombie spell, not the colored mana of the spell.
Death Baron will cost .
Geralf's Messenger will still cost
August 17, 2016 2:15 p.m.
Thank you both for your reply's. I understand it now fine.