Dakkon, Shadow Slayer is an awesome planeswalker from Modern Horizons 2 that seems to be extremely powerful. The unique loyalty ability forces us to reconsider how this card will be played.
Three mana planeswalkers tend to have a much higher success rate at being playable in competitive Magic than their more expensive counterparts. Let's first evaluate Dakkon's ability to be a useful spell on turn 3. Against an aggressive opponent, the floor is essentially just a sorcery speed Unmake, which doesn't seem too bad as a worst case scenario for your card advantage engine. Against an opponent who hasn't developed their board, you can simply plop this down, tick it up to 4 loyalty, and begin accruing advantage for the rest of the game. So far so good!
But of course, Dakkon only gets better later in the game. Even as early as turn 4, you can cast the Unmake version of Dakkon but have it stick around to either absorb some damage or stick around to keep providing value. This of course only gets better as the game progresses…imagine a drawn out game where your 3-cmc planeswalker enters the battlefield with 14 loyalty! Which brings us to our next topic…figuring out how to use Dakkon's "ultimate" ability.
Dakkon's final ability is not as powerful as some other planeswalkers' abilities that generally win the game, which makes sense since it's incredibly easy to be able to just drop Dakkon and immediately -6. But that leaves us asking ourselves how we can use this ability. It is nice that Dakkon's own surveil ability can contribute a large amount towards filling our graveyard with whatever large artifact cards we choose to run. Since we can put artifacts into play from our hand, it might be possible to just go big and try to cheat in something like Blightsteel Colossus. On the more conservative end of the spectrum, this ability might just be a little extra bonus in an Esper Stoneblade style deck where we could return a Batterskull that has hit the graveyard one way or another. Even if this last ability is just a small afterthought, the first two abilities make Dakkon a very exciting inclusion in any Esper deck!