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Henzell says...
  1. A summoning invocation from {demonspawn corrupters}. Summons 1-3 nasties at a time from Slime, Elf, V, Crypt, Tomb, Abyss, or Zot, with a summon cap of 6; examples include acid blobs, mummy priests, revenant soulmongers, or starcursed masses. Be wary.
  2. The wiki has a table showing off odds for all the monsters it can pull and their respective chances: 'http://crawl.chaosforge.org/Plane_Rend'

Information based on master branch and the great LearnDB.
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