Morphin' into an immortal coming through the portal
He is an immortal, referring to his long, thriving career and his godlike ability, coming through a portal from his godly realm. Marvel’s version of Asgard can only be accessed by a portal, through the Guardian of Worlds, the Gatekeeper, Heimdell. Considering Eminem’s fascination with comics (touched in Legacy), it’s likely he’s referring to himself in similarities to Odin.
He shared a video on his Facebook before this song dropped, named “Immortal’s Portal” by Patrick Cassidy. This is probably where some of his inspiration came from, for both the music and the video.
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