what are double sleeved cards sleeving questions : r/magicTCG 600 Pack Card Sleeves Clear,

Gym Leader Challenge is a popular example and is one of many listed in the Alternative Play Handbook linked below

While we have more than enough actual slivers to choose from, there are a few changeling cards that might be considered: Mutavault, Shields of Velis Vel, Amoeboid Changeling, Blades of Velis Vel, Shapesharer, Mirror Entity, Taurean Mauler, Chameleon Colossus

It copies all triggered abilities of the chosen creature type, not just ETBs, which opens up so many more interactions than the average Panharmonicon effect

Others, such as Dragon Star or Card Labo, also provide extensive table setups for players to duel with others, and often host their own tournaments and events to encourage players to meet and play

No one is quite sure how that happened, but you could get a 3/3 for three mana with a minimal drawback and the ability to hide from removal