In New Avengers, Dr. Strange summons some tentacle thing from a portal using a spell from the Blood Bible. It's so hardcore even Sun-God is severely weakened permanently.
New Avengers #21
What is that thing?
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Been also searching for an answer. Looks like one of the Marvel Elder Gods and/or The Many Angled One from the Marvel Cancerverse (which may actually hail from Earth-616). Cthulu and the Elder Gods exist in the Marvel and the Ultimate (Earth 1610) universe making appearances here and there
What he summoned isn't explained or likely to explained.