1/8
Many people like me use ghosts solely for the purpose of developing pieces by allowing them to move through them and then in the late game they become practically useless, in my experience, because the piece that was behind them is gone and they trade poorly with other minions. This piece is made solely for developing multiple pieces and allowing you to reposition them before you take something with them. Also base tier is very cost effective being the only zero cost minion that does not block movement. However, I believe the 0 cost is justified as it has significant drawbacks. Running this piece basically sacrifices a minion slot to develop a champion and the piece can be used against you as enemies that move through it also get enchanted. Also it has no attacks so it is very useless late game making it a niche use but powerful in the right armies. Despite this unit being very simple and unexciting I believe it would be an interest piece in the game's meta. Btw this piece should meet both challenges.
2/8

The mirror mage is a unit that could allow for some very interesting and unexpected plays and would be an interesting unit as it can reflect both allies and foes alike causing chaos across the battle. This unit could reflect a bunch of minions to the other side leaving one side defenseless and all for a rush breaking an opponents line. I believe this unit also meets the challenge criteria.
Edit: Fixed typos, got rid of link spam, and made the minion 1/8 arcane.
3/8

This unit allows you to summon base fireballs at 5 moral each and can only cast a few times before turning into one. This unit is balanced by the fact that summoning fireballs to kill minions is a terrible trade but allows you a material advantage and gives you control over the board. Also this unit meets challenge 2 and bonus challenge 2.
Edit: Range/Magic inconsistencies fixed
4/8

This unit's range 2 unblock able attack is balanced by the fact that it will retreat of threatened by an attack and its difficulty moving diagonally. However this unit allows for some tricky plays if used well with other units and should not be under estimated. Unit meets challenges 1 and 2 and bonus challenge 2.
5/8

This piece has been an absolute pain to balance. This unit is decidedly different than the lightning mage as it can cast its spell in a location then 5 turns later cast it again destroying the unit there. This makes the unit a powerful threat. This unit is less powerful than previous iterations but now I believe it has satisfied its theme if slightly strong.
6/8

This piece is made for countering high cost minions particularly pike men and can be very dangerous with its powerful promotion. Also its knight attacks can be very dangerous with high forking power (and the fact that by including them it went from being dqed to meeting two challenges). This piece despite its high attacking power can be completely shut down by a strong minion line and is primarily a late game piece due to its inability to attack in front of itself. However it could be used earlier to exploit any potential gaps in the enemy line or swap with an ally champion to develop them. This piece meets challenge 2 and bonus challenge 2,
7/8

This piece's powerful passive ability is balanced by the fact that it is very slow, colorbound (except for +++), and gets destroyed by minions. Also this piece becomes less potent against unblockable attacks and useless against magic having a blind spot diagonally from it. This piece also could have some difficulty cornering high value pieces due to not blocking movement. This piece meets challenge 1 and bonus challenge 2.
8/8

Well this is my last piece (maybe because I might not submit avalanchemage due to it being too much nuance) This piece is very powerful due to it being able to set up double threats fairly easily and due to the fact that it can make a piece incapable to move without causing the piece to be revived in its place. However this piece, despite dominating in the early to mid game is garbage in the endgame due to the lesser amount of pieces and its slow movement that causes it to leave vulnerable the piece it defends during its revival time. Also a lone pawn can completely stop it from reviving as long as it stays in the same location.