Prince of Persia fast travel: How to unlock
Prince of Persia sidequests: Where to find each
Prince of Persia Amulets: Powerful trinkets
Prince of Persia Sacred Archive puzzles: Each solution
If you're exploring every nook-and-cranny in Prince of Persia: The Lost Crown, you'll likely run into The Deserter while travelling through the Hyrcanian Forest for the first time. This cannon-wielding soldier hails from an earlier invasion of Mount Qaf but is trapped in a small cave just above the main path through the forest.
The easiest way to locate him is from the Wak-Wak Tree where you meet Kamil when you first enter the area. Drop down twice, immediately after the tree, into the room with three archways—wall jump near the entrance and hit the orange flower to activate the platform. This lets you hop across the three arches and slide into the soldier's cave where you can start The Deserter quest.
How to complete The Deserter quest
The soldier trapped in the cave asks you to find his lost brother-in-arms, but he doesn't give you any indication of how to track down this wayward comrade. Luckily, he's quite close by; continue through the Hyrcanian Forest via the main path under the soldier's cave and then drop down at the next available opportunity into a small area, shown in the screenshot above.
You'll know you're in the right place because on the far left you'll find some of those purple vines that can only be smashed with a charged attack, behind which is a room containing one of the soldiers you have to defeat for The Lost Warriors quest. Don't go in there right now, however.
If you jump up onto the archway near where you dropped down, you'll spot a dead soldier hanging from a rope and Sargon will comment, pondering how he got up there. Jump and attack to cut down the body, before searching it to find the Cannonball item. Now, you need to head back to the soldier in the cave, tell him about his comrade's grisly fate, and hand over the Cannonball.
The soldier will give you a Soma Tree Petal for your trouble, helping you progress towards a health bar upgrade, and he'll use the Cannonball you kindly provided to blast his way out of the cave and escape—job done.