Thud, Demonic Initiate

You may pay phyrexian mana ({p/u}{p/b}{p/r}) to cast
Thud. You may do this only once per turn.
Thud, Demonic Initiate's toughness is equal to
the number of demons you control.
{t}: Exile the top card of target players library.
If you do, you must play a card exiled this way
before your next upkeep if able. If you cast a
spell this way, pay life equal to its mana value
rather than pay its mana cost.