You may pay rather than pay the mana cost for spells you cast.

Enchant creature
All creatures able to block enchanted creature do so.

Look at the top five cards of your library. You may reveal any number of creature cards from among them and put the revealed cards into your hand. Put the rest on the bottom of your library in any order.

Raid — When this creature enters, if you attacked this turn, create a 1/1 white Warrior creature token.

Choose two —
• Return target creature card from your graveyard to your hand.
• Target player discards a card.
• Destroy target artifact.
• Kolaghan's Command deals 2 damage to any target.

Tap target creature. It doesn't untap during its controller's next untap step.
Rebound (If you cast this spell from your hand, exile it as it resolves. At the beginning of your next upkeep, you may cast this card from exile without paying its mana cost.)

Flying
Pay 1 life: Regenerate this creature.

Changeling (This card is every creature type.)
As this creature enters, choose a creature type.
Whenever another creature you control of the chosen type enters, put a +1/+1 counter on this creature.

Whenever you cast an instant or sorcery spell, this creature deals 2 damage to each opponent.

Vampires' Vengeance deals 2 damage to each non-Vampire creature. Create a Blood token. (It's an artifact with ", , Discard a card, Sacrifice this token: Draw a card.")

Flying (This creature can't be blocked except by creatures with flying or reach.)
Whenever you activate an ability of a Sarkhan planeswalker, this creature deals 1 damage to any target.

Return target creature to its owner's hand.
Draw a card.

Flip a coin until you lose a flip or choose to stop flipping. If you lose a flip, Fiery Gambit has no effect. If you win one or more flips, Fiery Gambit deals 3 damage to target creature. If you win two or more flips, Fiery Gambit deals 6 damage to each opponent. If you win three or more flips, draw nine cards and untap all lands you control.

Haste
Whenever a creature an opponent controls dies, put a +1/+1 counter on Yahenni.
Sacrifice another creature: Yahenni gains indestructible until end of turn.

When this creature enters, sacrifice a creature.
: Regenerate this creature.

Commander ninjutsu (, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand or the command zone tapped and attacking.)
Whenever a Ninja you control deals combat damage to a player, reveal the top card of your library and put that card into your hand. Each opponent loses life equal to that card's mana value.

: Add for each artifact you control named Powerstone Shard.

When this creature enters, it deals 2 damage to target creature.

Target creature attacks this turn if able.
Draw a card.

Sacrifice a creature. Rupture deals damage equal to that creature's power to each creature without flying and each player.

Search your library for an artifact card, reveal it, put it into your hand, then shuffle.

: This creature gains flying until end of turn.

Goblin spells you cast cost less to cast.
Goblins you control have haste.

Flame Spill deals 4 damage to target creature. Excess damage is dealt to that creature's controller instead.

Until end of turn, target creature or Vehicle becomes an artifact creature with base power and toughness 4/5.
Draw a card.

Destroy target creature with power 4 or greater.
Cycling (, Discard this card: Draw a card.)

Each attacking creature gets +1/+0 until end of turn for each nonbasic land defending player controls.

Each player can't cast more than one spell each turn.

Whenever an opponent casts a spell, you may draw a card unless that player pays .

, or , : Return target creature to its owner's hand unless its controller pays .

Escalate (Pay this cost for each mode chosen beyond the first.)
Choose one or both —
• Creatures you control get +2/+0 until end of turn.
• Creatures you control get +0/+2 until end of turn.

Equipped creature gets +3/+3 and has ward . (Whenever equipped creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays .)
Equip . This ability costs less to activate for each other Equipment you control. (: Attach to target creature you control. Equip only as a sorcery.)

This spell costs less to cast if you control a Wizard.
Counter target spell.

Flying, haste
At the beginning of the end step, sacrifice this creature.
Unearth (: Return this card from your graveyard to the battlefield. It gains haste. Exile it at the beginning of the next end step or if it would leave the battlefield. Unearth only as a sorcery.)

Whenever Varchild deals combat damage to a player, that player creates that many 1/1 red Survivor creature tokens.
Survivors your opponents control can't block, and they can't attack you or planeswalkers you control.
When Varchild leaves the battlefield, gain control of all Survivors.

When this creature enters, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.