It's simply a convenient spot to put your experience counters, which can be represented with dice, glass beads, or other small items
The original classic Liliana's Specter is the three-drop 2/1 flyer for everyone
Karumonix gives your whole army toxic 1, letting you win games quickly by stacking up poison counters
I would encourage you to take a look at each creature on this list and decide which is most appealing to you and your specific playstyle, instead of assuming that the one at the top will automatically be the best for you personally
I would imagine Sarkhan's Unsealing would need to be renamed, since it explicitly references a Magic: The Gathering character, and the Eldrazi Kindred Instant Not of This World may not be a good thematic fit, but a Fight Rigging card themed after the Wall Market fighting arena would make total sense
Rediscover the origins of iconic PlaneswalkersJace, Chandra, Gideon, Liliana, and Nissain Magic Origins