To get the ball rolling, here is my design idea for Mumm-Ra. He is a little complex and has a lot of moving parts with the design, but I think it incorporates most of what he can do and he is pretty scary.
NAME = MUMM-RA
SPECIES = UNDEAD
UNIQUENESS = EVENT HERO
CLASS = SORCERER
PERSONALITY = RELENTLESS
SIZE/HEIGHT = MEDIUM 4
LIFE = 6
MOVE = 3
RANGE = 3
ATTACK = 3
DEFENSE = 3
POINTS = 300-400?
INSTANTANEOUS TELEPORT
After revealing an Order Marker on this card and instead of taking a turn with Mumm-Ra, you may choose Mumm-Ra or one Unique Hero you control and place the figure within 6 spaces of their original space and take an immediate turn with that figure.
SARCOPHAGUS RETREAT
Any time Mumm-Ra would be destroyed and the Mystical Sarcophagus is unoccupied and on the battlefield, you may instead remove all but 4 wound markers from this card and place him in the Mystical Sarcophagus.
MUMM-RA THE EVER LIVING
Instead of taking a turn with Mumm-Ra, you may reveal the X Order Marker on this card to flip this card and then take a turn with Mumm-Ra The Ever Living.
MAGICAL DEFENSE
When Mumm-ra is attacked by an opponent's figure that does not have the Magical Defense special power, the most wounds Mumm-Ra can take for this attack is one.
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NAME = MUMM-RA THE EVER LIVING
SPECIES = UNDEAD
UNIQUENESS = EVENT HERO
CLASS = SORCERER
PERSONALITY = RELENTLESS
SIZE/HEIGHT = MEDIUM 5
LIFE = 6
MOVE = 6
RANGE = 6
ATTACK = 5
DEFENSE = 5
POINTS = 300-400?
ANCIENT SPIRITS OF EVIL
After Mumm-Ra the Ever Living attacks with his normal attack, he may attack one additional time. When Mumm-Ra the Ever Living casts a spell, he may cast the same spell again before discarding the spell card.
DWINDLING POWER
After taking a turn with Mumm-Ra the Ever Living, place a wound marker on this card. When Mumm-Ra the Ever Living would be destroyed, flip this card and use the Sarcophagus Retreat special power.
MAGICAL DEFENSE
When Mumm-ra the Ever Living is attacked by an opponent's figure that does not have the Magical Defense special power, the most wounds Mumm-Ra the Ever Living can take for this attack is one.
FLYING
SUPER STRENGTH
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NAME = MYSTICAL SARCOPHAGUS
UNIQUE DO
SIZE/HEIGHT = LARGE 3
FULL COVER
LIFE = 8
MOVE = 0
DEFENSE = 6
OCCUPANCY = 1
POINTS = 200?
UNDEAD HEALING
After revealing an Order Marker on the card of an Undead figure you control that is within the Sarcophagus you may reveal the X OM on that figures card to remove 2 wounds from their card. At the start of a round, if there are no wound markers on the card of the undead figure, you may place the figure on any space adjacent to the Sarcophagus, if possible. Figures may not exit the Sarcophagus by another other means except this power.
MAGICAL ENERGY DEFENSE
When the Sarcophagus is attacked by an opponent's figure that does not have the Magical Defense special power, the most wounds the Sarcophagus can take for this attack is one. After any figure within 4 spaces of the Sarcophagus inflicts a wound on the Sarcophagus, roll one unblockable die against that figure.