The Great Welkynd Stone
The third ingredient Martin needs is a Great Welkynd Stone, which will need to be retrieved from the Ayleid ruins of Miscarand. If you've travelled on foot between Skingrad and Kvatch, you've passed these ruins. It's possible that you've even explored them - so you may already have the Stone. If so, you can just skip to the end. The ruins themselves are no great shakes - goblins and skeletons for the most part on the first level and they're so busy fighting each other that you could probably slip through almost completely unnoticed. The Great Welkynd Stone is in the "Morimath" section of the ruins, but the button that lowers the gate blocking your way into that section is at the opposite end of the "Sel Vanua" section from the entrance to Morimath. Look for the button in the middle of the southernmost wall.
The Sel Vanua section is a bit harder as far as encounters go - leveled zombies - but still no great shakes. The third level, Morimath, is still leveled zombies. The only surprise is the King of Miscarand. He's a leveled Lich (slightly higher level than you), whose Staff of Ruin disintegrates your weapon. The King of Miscarand and his two zombie helpers will appear and attack once you take the Great Welkynd Stone. Running for it won't help much as a gate blocking the way back will close and lock (Very Hard), so you'll probably need to slug it out with them. He's got some spell resistances up, but is vulnerable to fire. And, of course, there is always the old stand-by of simply beating on him with a Silver or better weapon.
Once you've taken care of the King of Miscarand, you can snag his Miscarand Key (it's not necessary, but it saves you from having to pick a Hard lock). When you step on the pressure plate in his now-open tomb, you'll open a passage out and also open a hidey-hole containing a couple of chests and another zombie. No great shakes unless the King beat on you pretty badly. You'll run into a couple or three more leveled zombies on the way out, but it will eventually put you out near the entrance to Miscarand and the outside world.
Give the Great Welkynd Stone to Martin to complete the quest. Martin doesn't know what the fourth component he needs is, but he sends you off to speak to Jauffre to complete the "Bruma Gate" quest (if you haven't already done it) or the "Allies for Bruma" quest (if you have).