"Return the Slab" is a reference to an episode of Courage the Cowardly Dog, and became NLSS Jargon during the truncated 1/27/2014 stream.
"Return the slab or suffer my curse" originated in the Courage the Cowardly Dog episode "King Ramses' Curse," which aired in 2000, and has long since been memetic (though Northernlion did not understand the reference at first ).
The January 27, 2014 stream was truncated by an hour to save Northernlion's voice following his return from PDXCon. This, combined with Northernlion's jetlag and increasingly bizzare glitches in Family Feud 2010/Spelunky, led to a very strange stream. "Creepy" comments were popular throughout, e.g. "wake up NL, you're still on the plane" and "Ben drowned."
A particularly disturbing series of glitches during Spelunky led someone in the Twitch chat to say, "NL, return the slab or suffer the curse:" Twitch chat blew up with dongers -level "return the slab" spam, forcing the chat into slow mode for the rest of the stream and causing much confusion for the already-disoriented NL.
"Return the slab" (along with, occasionally, some of the other "creepy" comments) remains a popular comment in Twitch chat, Twitter, and Northernlion's YouTube comments, from NLSS hosts and viewers alike.