The Serling Hotel in Dead Island 2 is on Ocean Avenue, the final location you get to explore in the game. It's the safe house where you meet Dr. Reeds for the first time.
Like other safe houses in the game, the Serling Hotel is also home to numerous locked doors and safes. One such safe is in the hotel's reception. Here's how to get the Serling Hotel Reception key in Dead Island 2.
Where to find the Serling Hotel Reception key in Dead Island 2
You can find the Serling Hotel Reception key on a runner zombie called Serling Concierge. They appear on the first floor of the hotel in an area smeared with oil and a bunch of fuel tanks.
The Serling Concierge won't appear here until you have finished the main quest, "The Final Gauntlet." After you have visited the Serling Hotel safehouse and completed the aforementioned quest, come back to the first floor, and the named zombie should spawn.
Once you have killed Serling Concierge, loot the Serling Hotel Reception key from their corpse, and head back to the second floor and into the safe house.
The reception is on the opposite end of the safe house's entrance with the big Serling Hotels written on the wall. The safe is sitting right below the reception desk. Use the key to unlock the Serling Hotel safe and get the Superior Melee Mutilator mode.