Police said two men stormed into a house in Belvoir Street, east Belfast, between 1.30 am and 2.30 am on Tuesday.
The man was then struck several times “around the face with a baton/hammer”.
When the man fell to the ground, “he was kicked in the head and torso before losing consciousness”, police added.
Detective Sergeant McPhillips said: “This was a particularly vicious incident which has left one man in hospital with what could potentially be life-changing injuries.
“I would appeal to anyone who was in the area at the time and saw two men acting suspiciously to get in touch with detectives as soon as possible on the non-emergency number 101, quoting reference number 287 of 03/01/19.”
Alternatively information can be passed anonymously to Crimestoppers on freephone 0800 555 111.