Aegar, the Freezing Flame In the absence of Wizards or Giants - Furnace Host Charger is the only high-power one at common - it might seem like triggering Aegar, the Freezing Flame will be difficult
The latter one was just printed and then you can tap it with five mana to sacrifice to get a free discover 4, which is card value since you can draw it if it's not feasible unlike cascade 4, and then you can trigger your leader
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Of the 43 unique Planeswalkers, there are 29 humans, but really only 12 unique ones, and only one, Koth of the Hammer, is black
If the target creature is already tapped as the ability resolves, you will still put a stun counter on it