One of the standout NPCs in Demon's Souls, Ostrava of Boletaria is a character you can completely miss if you don't wander off the beaten path. Found almost entirely in Boletarian Palace, Ostrava has a storyline than spans all four areas of the Palace - though you need to go looking for him to trigger each encounter. The rewards for doing so are well worth the effort.
This guide will show you how to progress Ostrava's storyline in Demon's Souls. It's a simple enough process, but easy to mess up if you let Ostrava roam too much.
Step 1 - Gates of Boletaria
To progress Ostrava's quest line you first need to meet him. Proceed through the Gates of Boletaria area until you get through the fog door on the walls, just after a fight with a Blue-Eyed Knight. Go down the steps past the fog door and continue through the area, past the Dregling Merchant's little building, until you reach a set of scaffolding.
To proceed through the area you normally need to climb the scaffolding. Ignore it for now and go down into the courtyard to your left. This will lead you indoors, into what looks like an eating area. Go straight through this building, back outside, and down the pathway ahead. Just before you reach the far wall you'll see a thin alleyway on your left, and you'll hear cries for aid.
Go down the alleyway. Ostrava is here, standing atop a wall that's surrounded by hostiles. Kill the Dreglings - and the guards that come up behind you - to bring Ostrava down from his perch. He'll thank you with a Brass Telescope, then proceed to patrol the area, killing anyone he comes upon. After this you can find Ostrava back at the Nexus, sitting on the crossbeam over the ominous forcefield in the middle of the area.
A point of caution: You shouldn't let Ostrava roam until you've killed everything in the vicinity of his perch. If Ostrava sets out on patrol and runs into tough enemies there's a chance he will die, ending the questline prematurely. If this happens, make sure to check his body for the Mausoleum Key, or you'll lose it forever.
Step 2 - The Lord's Path
Ostrava is found next on the Lord's Path, unlocked once you beat the Phalanx boss. Immediately after the Lord's Path Archstone you'll come to a room full of Hoplites. Kill or ignore them as you see fit, then check the left side of the room. There's a ramp leading down to a portcullis... and Ostrava is on the other side, begging for help.
Proceed across the first section of dragon-guarded bridge. There's a guard tower here, and you can use stairs on the outside to enter the wall. Take a left once you're inside and go down the narrow passage as far as you can. You'll find one Dregling and, as you near the end, a small contingent of crossbowmen. Throw up a shield to block their bolts and kill the lot of them. Ostrava is waiting where you left him, and he'll reward you with three Dark Moon Grass.
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As before, Ostrava will patrol once he's out of the woods. Clear the area or he could be killed, ending the questline.
Step 3 - Inner Ward
Ostrava's next appearance is much later in the game. Once you've defeated one of the four non-Boletarian Archdemons you'll unlock the way to the Inner Ward, and early on your path will be barred by a closed gate up a long set of stairs. This forces you to take the long way around to the main path that leads to the boss fog.
Once you're on the path to the boss fog - it's a wide road with a ton of enemies waiting for you - you can double back to the closed gate and open it to create a substantial shortcut through the level. Assuming you've brought Ostrava this far he'll be on the other side of the gate, begging to be let in. Open the gate and he'll lead a parade of soldiers and Red-Eyed Knights. Kill them all.
Ostrava is gone when you finish up, but he's not far. Go back up the road until it veers right. Look to your left to find a small alleyway where Ostrava is hiding. He'll thank you for your help and give you a Pure Clearstone.
Step 4 - The King's Tower
Your final encounter with Ostrava is in Boletaria's last area. On the final ascent to King Allant you'll run into a dragon, guarding the front door with its fire breath, and just beyond an elevator. On the way up to the elevator you'll find Ostrava, doubled over. He's in a bad way, and in his last moments he'll give you the Mausoleum Key.
You're not quite done with Ostrava yet, though. Continue up the stairs and, on the walkway to the elevator, you'll see a Black Phantom. This is Ostrava, and he'll prove quite an opponent. You can run past him if you want, though beating the Phantom will earn you his Rune Sword and Rune Shield. Quest complete.
Step 5 - The Mausoleum
Or... maybe not? Ostrava's last words to you implore you to use the Mausoleum Key he handed over, and doing so is to your benefit.
Head back to the Gates of Boletaria and make your way up onto the first wall, mentioned above. Rather than heading towards the Blue-Eyed Knight, however, head the opposite direction. This path through a tower will take you to a Red-Eyed Knight which would have murdered you during your first visit. Kill the Knight, then use the Mausoleum Key on the doors the Knight was guarding.
Inside you'll find Old King Doran, and he's guarding the Demonbrandt. Beat Doran in a fight to earn the fabled sword. Doran does a fantastic amount of damage with his sword strikes, but he's slow and easily manipulated into not attacking if you draw him out of the Mausoleum. You don't need to kill him to get the Demonbrandt... though if you go the whole way you'll also get the Ancient King's Set. This will also lower your World Tendency by two, so avoid killing Doran if you'd rather not take the dip.
One last thing: You can reach the Mausoleum, and Old King Doran, long before you reach the King's Tower. To do this you'll need to get the Mausoleum Key from Ostrava early. This requires either letting him die to enemies as he patrols, or... killing him yourself. He'll drop the Key either way. Killing Ostrava has obvious consequences for your World Tendency, so only take this path if White World Tendency is not a concern.