Of all the boss fights in Elden Ring, Commander O'Neil's is one of the more straightforward. That being said, if you don't approach this stout soldier carefully, he can easily one-shot you as if he were Radhan. As part of Millicent's questline though, which itself has some pretty sweet rewards, you'll want to put this optional boss down. Here's where you can find Commander O'Neil and how you can beat him.
If you're just starting out in Elden Ring and don't even know who Millicent is, check out our guides on what you should do first in Elden Ring and where you can easily farm runes early in the game.
If you're not a fan of the toxic swamps found throughout the Souls series, I've got some bad news for you about where O'Neil is located. He's straight in the middle of the Aeonian Swamp, in a location called the Heart of Aeonia.
Just reaching O'Neil is half the battle. You'll likely first approach the swamp from its west side, far from the Heart of Aeonia. Don't ride straight through the swamp to the site of the boss fight. You'll end up being invaded by Millicent, who will likely knock you into the swamp itself, and kill you soon after. Instead, circle around the swamp's north side and look for the Sellia Under Stair site of grace. From there, you'll be able to travel southwest to the site of grace just north of the Heart of Aeonia, called Inner Aeonia.
From here, travel southwest, around the Heart of Aeonia. You'll eventually find a small patch of land with a Stake of Marika and a summoning pool effigy. This is where you'll want to run into the boss fight from, as you can easily summon other players (although it's not the best choice) and run straight back in if you die.
Once you start the boss fight, things are somewhat
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