Nexus

At the beginning of your upkeep and nexus has
not been cast you may play nexus without
paying its mana cost
When you play nexus create 5 1/1 Probes
{1}{t}, Create a 1/1 probe token with "add the beginning of
your upkeep add {u}{w} to your mana pool"
Eminence—Mana does not empty out of you
mana pool and you cannot play land cards