This build is incredibly fun and surprisingly effective. Each item is a new powerspike -- Sated devourer procs your W in 2 autos, Frozen Mallet permaslows off your autos, Tri gives speed and more burst, BOTRK gives ridiculous damage and sustain allowing you to 1v1 just about anyone.

This build gives you an incredibly quick and high damage Gnar great at kiting and dueling. You are an ad carry with more utility and you are far scarier until the late game comes around.
Why devourer???? - After two autos your W is activated <

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this is the point of the build. You can AA twice, providing great damage, slowing the enemy with frozen malle, and becoming incredibly fast, and the best part is you can do it again immediately over and over!

-- You are NOT tanky with this build, don't play like it!
-- Mini gnar is your most DPS effective form and is great for skirmishes, whereas mega gnar is not great for skirmishes but is only really good when your team is with you and you can land some CC
-- You are weak clearing early game, ask team to ward your opposite buff that you are starting on. I recommend starting Krugs but Krugs or Gromp will work.
-- Have Fun!