At the very bottom of Brume Tower sits Sir Alonne's armour. If you've defeated the Fume Knight in the Crown of the Old Iron King DLC and obtained the Ashen Mist Heart from the Ancient Dragon in the Dark Souls 2 main game, you can examine the armour and enter a special memory. Inside, you'll face the supreme samurai Sir Alonne.
Sir Alonne wields a large katana, which means two things: heavy damage and a long recovery. Block or dodge his katana combo, and you'll have a moment after his attacks to land a few blows yourself. He will also slash horizontally or stab his katana downward, but these can be easily dodged with rolls.
Watch for Alonne's lunging attacks too--a simple roll to the side gets you in perfect attack position. Be aware that one of these attacks drains your health to buff his sword. Again, make sure to roll away from any lunges.
Finally, watch for any time dark energy channels through Alonne's blade--this means he will fire a bolt of dark energy your way. You can block this with a shield to take a tick of elemental damage, or use a well-timed roll to avoid the shot.
Sir Alonne isn't the toughest boss of this DLC, but that doesn't mean he's easy. Watch his attacks, dodge, and get in hits when you can to come out victorious.
For more help be sure to check out our complete Dark Souls 2 Boss guide.