Collapse Portal

This rare magic can seal the crystal sphere in which a fantasy world floats. If create portal spell was cast during the past turn within a one mile radius, the caster must make a Will save DC 20 to successfully cast collapse portal. If the check is successful, the crystal sphere is sealed.
If ship is at the sphere's shell as the spellcasting is completed, roll a d10 to determine when the sphere closes (see table).

1D10	Results
1–2	Portal closes before the ship reaches the sphere; ship must turn or ram the shell.
3–5	Portal closes as the ship reaches the sphere; ship unavoidably rams the shell.
6–9	Portal closes on the ship, cutting in cleanly into two pieces.
10	Portal closes just after ship passes through.

    A ship ramming or being cut by a crystal sphere sustains both collision damage and spelljammer shock (the helmsman must save vs. spell or die).
    A ramming ship suffers 20d10-hit points of damage (a ship with 0 hit points breaks up). The ship rings with impact, and all those not sitting or firmly tied down have a chance to loose their balance and fall. Everyone must make a Reflex save DC 15 or fall prone.
    A bisected ship also breaks up immediately, its atmosphere dividing between the two parts of the ship; decide randomly where the break occurs. All beings at risk should make a reflex saving throw DC 15 to avoid being slain by the closing shell, and any items at risk must also make a reflex saving throw to avoid destruction.
    Few know how to cast this spell, though almost every spacefaring being has heard of dark tales of it cutting ships in half as a crystal sphere closes.
    Arcane Material Component: Two smooth stones of any size, one of diamond and the other of obsidian.

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©2002 Wizards of the Coast, Inc.

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Collapse Portal, © 1990 Wizards of the Coast, Inc.
d20™ System, Conversion of Collapse Portal ©2002, Mark Doolan
Original Source: Lost Ships