Boundless, the

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The Boundless is a peculiar Demiplane containing endless crystal strands, spires of spinning vapor, and a gelatinous ocean in which mysterious forms swim. It heals the wounds of any who enter it for the first time, restores youth and vitality to those who enter it a second time, and permanently traps those who enter it for a third time.

See the Rule+of+Threes.

Source

A Guide to the Ethereal Plane

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