I rather like this fellow. A good head on his shoulders, concerned about his guild members, well-spoken, everything a guild steward should be. You may have already talked to him to get advice about other quests. If not, you should. Unfortunately, he won't have any quests for you to do until you are at least Rank 3 (Swordsman) in the guild and his disposition toward you is at least 30.
Your first task is simple enough: help a Buouyant Armiger named Ulyne Henim deal with a necromancer. You are to meet her in front of Vas. Finding the place is the hardest part, but Mercius should be kind enough to mark it on your map for you. If not, it's on one of the larger islands west of Dagon Fel, kind of northeast of the Valenvaryon stronghold. Anyway, meet Henim at the entrance and ask her to follow you. Henim is a decent enough fighter, so she'll pitch in and help where needed.
Vas is a Velothi dome and consists of an entry level (3 NPCs and six leveled undead critters) and a tower (the necromancer, a couple of leveled dremora, and a couple of leveled undeads). Bear left once you're inside and go to the lower floor, where you'll find the entrance to the tower. Henim should stop following you once the necromancer is dead, so if you bypassed anything on the way in, you will not have the help on the way out.
If Henim does most of the work, you'll get 250 gold, 5 faction rep points and 10 disposition points for Mercius. If you do most of the work, you'll get 500 gold, 5 faction rep points and 15 disposition points for Mercius. If Henim doesn't make it out alive, you get squat and Mercius will lose 10 disposition points.
A bounty-hunting job. Nerer Beneran used to belong to House Redoran, but is now an outlaw and needs to be hunted down. He's holed up at Sargon, north of Maar Gan. Cruddy directions. It's on the big island almost directly southwest of Vas.
Anyway, Beneran and his little gang (7 of 'em, with a skeleton champion thrown in for spice) are fairly scattered throughout Sargon. Most are by themselves and Beneran is way at the back of the place (couldn't one of these guys be right next to the entrance for once?).
Once Beneran is no longer an issue, report back to Mercius for your reward: 500 gold, 5 points of faction rep with both the Fighters Guild and House Redoran, and a 10-point disposition increase for Mercius.
Serjo Avon Oran has hired the Fighters Guild to take care of a group of bandits led by Daldur Sarys. Mercius sends you to talk to Oran for the details. So, hop on over to Suran and find him at Oran Manor. If you killed Oran before this point, you will not be able to get this quest. Oran tells you that Sarys and his bandits are holed up in Saturan, which is just over the mountain, northeast of Suran. The directions are accurate (believe it or not) and the place isn't too difficult to find.
It's a straightforward bandit cave. You'll have to go through four or five of Sarys' men to get to him (he's at the back, wouldn't you know). When the walkway comes to a tee, go left or you'll find yourself in a massive brawl with seven bandits. You don't have to take out everyone unless you really want to or you went right instead of left.
Report back to Oran for your reward: 1000 gold, a 5-point faction rep increase for both the Fighters Guild and House Hlaalu, and a 20-point disposition increase with Oran. Reporting back to Mercius doesn't actually do anything except clear the way for the next quest.
Report to Dangor (a Bosmer) at the Elith-Pal mine with a load of Flin for the miners (ebony mining must be thirsty work). The mine is almost directly west of the Zainab Ashlander camp at the base of the mountains. Aside from overland encounters, there isn't anything to fight here. Just deliver the flin and report back to Mercius.
When you report back, you'll get 500 gold, a 5-point Fighters Guild faction rep increase, a 5-point disposition increase with Mercius and he'll offer you some brandy. If you accept, you'll get a bottle of Cyrodillic Brandy.
You must be at least Rank 8 (Guardian) before he will offer this quest and there are a couple of ways that you can play it out.
Although Mercius' quests have stayed away from it, the current leadership of the Fighters Guild has more or less allied the guild with the Camona Tong and you may have already had quests that involved putting the screws to the Thieves Guild. Mercius thinks it's time for management to undergo a radical overhaul and he sends you out to kill Eydis Fire-Eye at the Balmora Fighters Guild and Lorbumal gro-Aglakh at the Vivec Fighters Guild.
If you are a member of the Thieves Guild and have completed the Bitter Bribe quest, Eydis will no longer be an issue. If not, then you'll have to either take her out or do the Bitter Bribe quest. Remember, there is no time limit on this quest once you have accepted it, so feel free to join the Thieves Guild, do the Bitter Bribe, and then report back to Mercius to have Eydis removed from your hit list.
Once you have accepted this quest, and assuming that you have not done the Bitter Bribe, Eydis should go hostile on you as soon as she sees you (which lets you kill her without getting kicked out the guild). Whatever you do, don't attack her until she goes hostile or you'll get expelled from the guild and all of the other guild members on Balmora will go hostile on you, too. Eydis is 19th level, has her weapons and armor skills in the 60's and 70's, and has about 250 health. She's sporting a full set of bonemold armor and a Dwarven Mace.
For Lorbumal, however, there is no work-around. You will have to take him out. Like Eydis, he will go hostile on you when he sees you while this quest is active. He's 19th level, has the skills to support it (weapons and armor skills in the 60's and 70's) and has 200 health to back it up. He doesn't have armor (and no script to add it), but he does have a couple of Dreugh clubs.
I have run into the situation where I had opened Sjoring Hard-Heart's door at some point and my fight with Lorbumal brought him into the fray. Fortunately, I was packing some serious magic armor and weaponry and was able to take them both on, but they put a majorly serious hurt on me before it was all over.
Report back to Mercius after this quest is complete. There is no reward for this quest except being able to keep whatever Eydis and Lorbumal had on them.
Your next (and final) mission is to kill Sjoring Hard-Heart, the current Guild Master. Mercius will offer you some armor, which you would do well to take. Hard-Heart is 23rd level, but his combat skills are down in the 50's and 60's. Of course with 100 STR, 100 SPD and close to 250 Health, you don't need to hit often in order to put a bad hurt on someone. Hard-Heart is sporting a full suit of Steel armor and a magical warhammer.
When you report back to Mercius, he'll promote you to Master (actually you get two ranks out of him, but you should not have received either of the last two quests without being at least Rank 8) and urge you to run the guild wisely and well.