Using the Runes of Summoning, summon and subdue a voidwalker, then return the Runes of Summoning to Carendin Halgar in the Magic Quarter of the Undercity.
Surely you have seen the circle of summoning below us? Only there will you be able to draw forth an unbound voidwalker.
Using these runes, you will be able to draw it forth, and then you must dominate it through strength of arms and magic.
When it is subdued, it will be indentured to you. Come to me then, and I will elucidate the spell of summoning, so that it may serve you to whatever end you wish.