The warlord Roderick must die. He's at Fort Sutch, is gravely ill and is being kept alive by a miraculous medicine of some kind. It's not enough that he simply stop living. His death needs to be seen as the natural result of his disease. So your job is to sneak into Fort Sutch, swap out Roderick's medicine for a bottle of poison that Ocheeva will provide, and sneak back out again. You cannot be seen or everyone's guard will be up and the poisoning will fail. You can kill him "the old-fashioned way," but if you want the bonus, don't be detected and use the poison.
If you don't care about the bonus, the quick and dirty way of doing it is to travel to Fort Sutch (it's north of Lord Drad's Estate, which is north of the point on the road to Anvil where the Highwayman jumps you), enter through the front door and fight your way to Roderick's bed where you can bonk him and be done with it. There are a couple or three fighter-types who will probably get in the way, but nothing that a good combat-oriented character can't handle.
But if stealth is your thing, poll the Sanctuary. Teinaava will be able to tell you about a secret tunnel that runs from an abandoned abbey a little way away from the fort into the fort proper (which will make it appear on your compass pointer). He makes it sound like you'll need to swim to get in, but you don't.
Use the tunnel to get into the fort. Be careful as you come up out of the water, though. There is a patroling guard (a Wood Elf named Laenafil) who comes down to near the water's edge and then patrols back into the fort. She's a level 1 Warrior and her Sneak skill is almost nonexistent, so slipping by her shouldn't be much of a problem unless your Sneak skill is abyssmally low, too.
The major problem is going to be that you can't get through a key-only gate and the only other way in is blocked by Neesha and Ulmog gro-Cromgog, two of Roderick's lieutenants. Sneak up and listen in on their conversation. When they are done talking, they're going to come your way. HIDE!!! After they have passed, follow them because they are going to open the gate that you couldn't get through earlier. Neesha will head for Roderick's bedside (in case you decide to go for the "old-fashioned way") and gro-Cromgog will head off to guard the Medicine Cabinet. Neesha is a level 1 Warrior and gro-Cromgog is a level 5 Warrior. Neither of them should be terribly difficult to take down if things get hairy, but we don't want to let them get hairy to begin with.
gro-Cromgog does a patrol route around a couple of big, thick support pillars near the Medicine Cabinet. If you're right behind (or reasonably close) when he reaches the proper area, you can get to the Medicine Cabinet and do your thing before he even has a chance to notice you. Remove Roderick's medicine, replace it with the poison, and then avoid gro-Cromgog on your way out again. You can either go out the way you came in or head for the front door. If you choose the front door, you'll need to pick the lock on a gate or two, but it's a pretty straight shot and the front door will be immediately to your left after the second gate. There is an NPC named Gerard in that area, but he tends to stay put and shouldn't present a problem.
Report back to Ocheeva. You'll get your usual contract gold and (if you were undetected) a robe called "Deceiver's Finery" which has a leveled Fortify Personality and Speechcraft enchantment. It might come in handy for the next mission.