The Storm King is one of Demon's Souls' really interesting bosses that has a really cool mechanic to use against him.
After entering the boss fog, run down to where all the Storm Beasts are. Head towards the cliff on the left and you'll see a sword in the stone.
Demon's Souls - How to beat Storm King
|Demon's Souls | Storm King Weakness|
Grab it to obtain the Stormruler, a unique weapon that summons the power of the storm with every swing. It only works inside this boss room, so enjoy it while it lasts!
With the Stormruler in hand, you’ll be able to attack the Storm Beasts and King without needing any other ranged attacks. Of course, if you have arrows or magic, you can use those instead. The Lava Bow and Soul Ray are particularly effective against him. After you kill six Storm Beasts, the Storm King will begin to attack.
The best spot to kill the Storm King safely is the broken house, as it covers you from every angle.
Stand inside the house and wait for the barrage to end. Then, run out and attack him until he’s out of range. Repeat this as he goes back and forth on his attack runs and the Storm King should be easily defeated.
After defeating the Storm King, you will receive the Storm Demon’s Soul and Pure Cloudstone. The soul can be used to create the Morion Blade at Blacksmith Ed or learn Anti-Magic Field from Saint Urbain.