Cathedral Of Blue:

The Old Dragonslayer is weak against Lightning and Magic, and can be easily poisoned.

Keep your shield up at all times and constantly circle around the Old Dragonslayer's left side to avoid most of its thrusting and lunging attacks.

Be prepared for occasional sweeping attacks, which can be absorbed by your shield.

Warning: When you see a dark purple cloud forming, roll away quickly. The Old Dragonslayer will rise and slam down, releasing a potent magic blast in a ring outwards. This attack deals heavy damage even with a shield.

Evade the numerous small magic homing missiles the Old Dragonslayer can also use.

Casters and archers should use the same strategy: get in close and circle the left side to create openings for spells at near point-blank range. Alternatively, maintain distance and be prepared to dodge frequently or use a high block rate shield.

After defeating the Old Dragonslayer, you will receive 20,000 souls, the Old Dragonslayer Soul, and the Old Leo Ring.

Head through the next door and loot the two chests to receive three Cracked Blue Orbs, the Heide Knight Iron Mask, and a Tower Shield.

You will meet Targray of the Blue Sentinels.

To join the Blue Sentinels covenant, bring Targray a Token of Fidelity.

To obtain a Token of Fidelity, you must help another player defeat a boss as a phantom.

Locate the staircase where Targray is standing and head down to light the Cathedral of Blue Bonfire.