Tendrils of Corruption deals X damage to target creature and you gain X life, where X is the number of Swamps you control.
The amount of life you gain is equal to the number of Swamps you control, not the amount of damage Tendrils of Corruption deals (in case some of it is prevented).
If the targeted creature is an illegal target by the time Tendrils of Corruption would resolve, the entire spell doesnâ€™t resolve. You wonâ€™t gain any life.
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