It’s for the mage combo. You use Simulacrum to copy Sorceress Apprentice and discount them with Leyline Manipulator. Then the combo is:

3 Sorceress Apprentice (1 regular and 2 copied), 2/10 mana
Archmage Antonidas, 9/10 mana
Molten reflection, 10/10 mana
Infinite fireballs for 0 mana to end the game.

Thanks mate! Will update the description!

The main reason you run 2 of Mysterious Challenger is redundancy. While the second Challenger is weaker than the first one, the first one might be insane enough to win the game. Therefore you want to run 2 copies just so you draw into the first one earlier. It’s basicly the same situation as Ultimate Infestation, While running one copy might be better in theory, in practice it makes the deck less consistent since you’re less likely to have Challenger on curve. I haven’t played the deck much, but I would replace King’s Blessing for N’zoth. N’zoth might also be used if you lack Dr. Boom as it provides stronger boardswing, but is both more expensive and requires some setup.

Look pretty fun to use with Control Shaman. It’s a nice combo with Earth Elemental at 10 mana.
It’s basicly an ancestral spirit (2 mana) bundled with a mediocre 3 mana statline. All in all, a pretty fair card since ancestral spirit is a class card and this is a neutral.

And here I was predicting this effect for the DK Priest Hero Power…
Anyway, this card gives Priest some more OTK options as long as you play less than 3 minions and Velen died.

2x Radiant Elemental (4 mana, 2 cards) + 2x Eternal Servitude (8 mana, 4 cards) + 2x Mindblast (8 mana, 6 cards, 40 damage)
Not really a practical OTK, but I’m going to enjoy messing around with it in friendlies!