Guilds of Ravnica

Erratic Cyclops | Creature | Cyclops Shaman | Trample Whenever you cast an instant or sorcery spell, Erratic Cyclops gets +X/+0 until end of turn, where X is that spell's mana value. | Milivoj Ćeran

Legion Warboss | Creature | Goblin Soldier | Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.) At the beginning of combat on your turn, create a 1/1 red Goblin creature token. That token gains haste until end of turn and attacks this combat if able. | Alex Konstad

Runaway Steam-Kin | Creature | Elemental | Whenever you cast a red spell, if Runaway Steam-Kin has fewer than three +1/+1 counters on it, put a +1/+1 counter on Runaway Steam-Kin. Remove three +1/+1 counters from Runaway Steam-Kin: Add {R}{R}{R}. | Jason Felix

Swiftblade Vindicator | Creature | Human Soldier | Double strike, vigilance, trample | Viktor Titov

Tajic, Legion's Edge | Legendary Creature | Human Soldier | Haste Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.) Prevent all noncombat damage that would be dealt to other creatures you control. {R}{W}: Tajic, Legion's Edge gains first strike until end of turn. | Anthony Palumbo

Niv-Mizzet, Parun | Legendary Creature | Dragon Wizard | This spell can't be countered. Flying Whenever you draw a card, Niv-Mizzet, Parun deals 1 damage to any target. Whenever a player casts an instant or sorcery spell, you draw a card. | Svetlin Velinov

Chamber Sentry | Artifact Creature | Construct | Chamber Sentry enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it. {X}, {T}, Remove X +1/+1 counters from Chamber Sentry: It deals X damage to any target. {W}{U}{B}{R}{G}: Return Chamber Sentry from your graveyard to your hand. | Adam Paquette