Definitely do this, the prize isPer dear Astrid's orders, I found myself in Markarth on a mission for the Dark Brotherhood. I've spent a fair amount of time in this Hold, even was imprisoned here on false charges after gaining the attention of the wrong people for investigating a murder in the streets. I've skulked the alleys of the city of stone by day and night in search of blood and silver. Yet nothing could have prepared me for my greatest challenge yet.
The servant of Mara, Dinya Balu, had tasked me with spreading sweet love throughout Skyrim. In Markarth, I was to find the old mage and codger Calcelmo and assist him with wooing his true love! She turned out to be Faleen, the hardened Redguard Housecarl to the Jarl himself! This was truly a worthy challenge for the Guild Master of the Thieves Guild and hardened assassin of the Dark Brotherhood! With some difficulty, I was able to track down a bard by the name of Yngvar. With a little persuading, I convinced him to write a fake love letter in the form of poetry purporting to be written by Calcelmo's hand. I found Faleen by the Jarl's side and delivered this truly lewd and eloquent Poetic Edda of Love to her hand. She bade me return an even more indescribably lewd letter to Calcelmo, and I witnessed as they met and professed their love for each other! By Mara! I may be a thief and assassin, I am the Dragonborn in whom the fate of the world rests, but above all, I am a wedding planner!
If you haven't done this quest, you can get it at the Temple of Mara in Riften. The quest is called "The Book of Love".
15% resistance to magic.