I like the Mummy/Pharoah concept, and I also like Arondight's Reinfield idea, so I thought I'd throw out some others.
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Wood Golem becomes
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Fire Golem when targeted by Firelance/Firestorm
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Fire Golem becomes
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Ash Golem when targeted by Ice Bolt.
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Ice Elemental becomes
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Steam Elemental when targeted by Firelance/Firestorm
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Steam Elemental becomes
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Ice Elemental when targeted by frozen/congealed
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Corpse becomes
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Frankenstein when targeted by Lightening
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Clockwork Golem becomes
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Robot when targeted by Lightening
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Ghost becomes
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Reborn Soldier when targeted by Holy Light
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Plagueling becomes
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Giant Rat when targeted by Holy Light
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Giant Rat becomes
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Plagueling when targeted by Alphatoxin
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Sand Elemental becomes
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Mud Golem when targeted by freeze/congeal
Dr. Jeckle
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becomes
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Mr. Hyde when targeted by Rage Elixer
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Flesh Golem becomes
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Stone Golem when targeted by Stone Skin
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Chaos Mist becomes
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Iron Sphere when targeted by Gravity Pull.
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Iron Sphere becomes
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Chaos Mist when targeted by Chaos Seed/Chaos Power
This concept opens a large dynamic to the elements game. These are just ideas to get us all thinking.
Jallen