This achievement is tied to one of the two possible endings to the game that you get to choose after defeating Gwyn, Lord of Cinder in the Kiln of the First Flame.
In order to access this final area you must acquire the Lordsvessel from Anor Londo, place it on the Firelink Altar, and then defeat the four main bosses- Seath the Scaleless, Gravelord Nito, the Bed of Chaos, and the Four Kings- and offer their souls to the Lordvessel.
The linear path through the Kiln of the First Flame has five respawning Black Knights that can be farmed for their weapons/shield and for various titanite chunks. Solaire's summon sign can be found about halfway through this area after you cross a makeshift bridge IF you progress his questline properly.
These are the steps that I follow from the start of the game to make Solaire summonable:
1) Meet Solaire in the Undead Berg after the Taurus Demon. Exhaust his dialogue and answer "yes" to receive the White Sign Soapstone. His summon sign should be available at the Bell Gargoyles and Gaping Demon. I always summon him when it's possible, but it might not be necessary.
2) Talk to Solaire again in Anor Londo at the bonfire that follows the two Greatbow-wielding Silver Knights on the ledge of the main building. His summon sign should be available for Ornstein and Smough on a small platform near the Silver Knight Archer in the giant room preceding the boss fog door and then much later on outside the Centipede Demon fog door in the Demon Ruins.
3) After defeating the Centipede Demon, Solaire will appear by the bonfire in the next area, but I never go in there to talk to him. I instead homeward bone after defeating the Centipede Demon and then warp to the Daughter of Chaos bonfire and offer 30 Humanity to Quelaag's Sister, which gets you to rank 2 in the covenant. This allows you to open the shortcut door to Lost Izalith that can be found in the path to the right of the Demon Firesage's boss area. This area contains nine Chaos Bugs, one of which has red eyes and always drops the Sunlight Maggot. You MUST kill this particular Chaos Bug (I always kill all nine to be safe) before entering Lost Izalith to prevent Solaire from going insane. Once these Chaos Bugs are dead, Solaire should appear near the shortcut door (quit and reload your game or use a bonfire and then return if he doesn't appear). Talking to him here makes it so that his summon sign will be available in the Kiln of the First Flame.
If you enter Lost Izalith the proper way after the defeating Centipede Demon without having killed the special Chaos Bug, Solaire will go insane and you will not be able to summon him for the Gwyn battle.
Gwyn is an extremely aggressive boss that is difficult to defeat solo. Having a partner against him changes the dynamic of the battle completely and you can't count on being able to find a random human player to summon, so completing Solaire's questline will make your life infinitely easier.
If you do have to fight Gwyn solo, be prepared for a brutally tough battle. I suggest using a Black Knight Shied as it has great Fire reduction, 100% Physical reduction, and solid stability. Try to kite Gwyn around the circular rock formation on the left side of the arena as fighting him on open ground is very difficult. Avoid or block his attacks and counterattack when you can. You'll have to play defensively and chip away at his health as going toe-to-toe is not really an option. It's often difficult to find enough time to heal or to just let stamina regenerate as his attacks are relentless.
Note that some of his attacks can be parried but this requires a lot of practice and precision.
Once you defeat Gwyn, a bonfire will appear in the arena. Ignore this bonfire and walk out of this arena to become the Dark Lord and the achievement will unlock.
IMPORTANT: defeating Gwyn ends the playthrough and moves you on to the next one, so make sure you have done everything you want to do before fighting him.
As always, refer to http://darksouls.wikidot.com
for specific information relating to items or locations.