The attack happened in the car park of the Red Bull Sports Bar in Whiteheath, Oldbury, on Sunday evening.
The man, aged in his 20s, is believed to have been beaten with golf clubs, pool cues and wooden poles.
The car park and pub in Birchfield Lane were sealed off as police forensic teams examined the area on Sunday night and remained closed on Monday morning.
So far no arrests have been made.
Officers marked up to 20 pieces of evidence with yellow forensic cones in the car park, where broken pool cues had been left discarded near a golf putter, wooden poles and a black trainer.
Towels covered in blood were also left in the car park as police and paramedics were sent to the scene at around 5.50pm.
West Midlands Ambulance Service said they treated one man who was taken to hospital in Birmingham with a serious head injury.
He remained in a critical condition on Monday.
Another man was discharged at the scene with minor injuries.
The Red Bull Sports Bar and Grill, formerly the Bulls Head, is on the A4034 Birchfield Lane near its junction with Throne Road, around 600 metres from Birchley Island.
Anyone with any information is asked to contact police via Live Chat at west-midlands.police.uk or by calling 101 anytime.
Quote crime reference 20SW/284067Q/19.