the first question is do we want this creature to be Legendary or not?
First there is the creature who feeds on dreams. This would be like a kick ass version of haunter from pokemon. It puts you to sleep and then feeds on you without breaking the condition. Then, there is the "Phantom Force Nightmare". This one could be like a poltergeist or some form of intangible force knight with a horrifying presence. And I'm sure you guys have ideas for this as well. We're close. After this step I'll build some playtest cards for us to see just what we've come up with.
Personally, I'd prefer non-legendary. While a Legendary creature allows for more unique abilities, the mind school kind of needs solid mid-tier soldiers to deploy. That being said, I do have an idea or two for a legendary creature as well. Disclaimer: these are all initial ideas, and have had absolutely no thought given to balance concerns yet.
1. Mind-Consuming Darkness
Idea = a dedicated anti-familiar. Consumes them and grows more powerful.
Specifics = attacks gain Unavoidable and +2 against all familiars. If a familiar is killed by this creature's attack, its card is placed under this creature's card. These creatures cannot be resurrected or regained in any way by their mage. For each captured creature card under it, this creature gains Reconstruct +1, Melee +2, and an extra action marker (giving it an extra turn). Whenever this creature takes damage, its controlling mage may discard one of the creature cards under it to regain health equal to the number of health listed on that card. Afterwards, that card is Obliterated.
Notes = the upside is that it's very different from any other existing creatures, and it suits the theme fairly well. The downside is that its abilities would only suit a legendary creature, and familiars aren't extremely popular or prevalent anyway (meaning the card would be HIGHLY situational).
2. Haunting Shadows
Idea = relatively weak creature which enemy mages are forced to deal with personally. Sort of like living homing missiles.
Specifics = this creature may only move in the direction of the nearest enemy mage. It cannot be targeted by any non-mage creatures or conjurations. It is Obscured, Unstoppable, and Uncontainable. It cannot attack anything except a mage, and its attacks are Unavoidable and Critical. It is unaffected by terrain and walls.
Notes = upside is it doesn't have to be legendary, is somewhat unique, and fits the theme of a "phantom force nightmare". Downside is that it will be challenging to make this idea scary instead of annoying for an opposing mage while not having it be too powerful.
3. Nightmare Illusionist
Idea = a solid mid-tier creature which can make damaging illusions of itself, making the enemy guess as to which one is real
Specifics = this card comes with 4 tokens. all tokens possess the same face-up side, and 3 of them say illusion on the other side. The last token says real on its face-down side. When this creature is summoned, place all 4 tokens in areas within 1 space of the controlling mage (can be placed in different spaces from each other). Place the creature card next to the mage's equipment. Each token possesses its own action marker and takes its own turn. Whenever one of the tokens is attacked, it is flipped over. If the token says real, the creature card is placed in that space and all illusion tokens are discarded. If the token says illusion, that token is discarded. Each token may attack and deal damage as normal, but only the real token may damage psychic-immune creatures.
Notes = on the upside, this would give the mind school a solid soldier card, while also incorporating the theme of mind games. On the downside, this might prove unwieldy and intrude too much on whatever is planned for the Siren (or a future Illusionist). It also relies on psychic attacks by necessity (which might be a problem).
I might think of other ideas later.