Im sitll only lvl 1, and I lost a bit of health, but I have no clue why I was attacked! Anyone know a possible cause?

Tribunal expansion.

Tribunal Expansion
if you want more of an explanation ask in the spoiler section.

It is the start of the Tribunal expansion. Apparently the expansions are aimed at much higher level players, so ignore it (like I am) until you get to a much higher level.

Oh, ok thanks.

Those dark assassins are like that... sneaky and can't be trusted. Sleep with your sword in your hand.

Console versions pushed the attacks back until you had gained several levels, not sure of the exact numbers. Attacks are random. Testing it, I once slept 13 times in a row before being attacked.

I'm playing through vanilla right now because I can't resist the urge to just get the awesome assassin armor the first moment I can sleep.

Welcome to Morrowind! You'll know the cause when you'll investigate further into the matter.

There's a mod for PC players that pushes back the start of this, as well.

Mind, if you've got a few levels and a decent weapon, it's a good way of getting loot. And you'd think your attackers would figure out the same methods clearly aren't working despite being applied time and again, but no...

It's not like they have a choice... It's either that or no money...

But think of the waste in manpowe...humanpowe...um, species power! In time! In weapons and outfits! They might as well cast a Drain Financial Resources spell on themselves.

You dont expect them to start using invisibilty or chameleon, do you? Or sending multiple assassins at once? I mean, that would be too logical.

Back when I had my last character, I decided to build a pile of loot salvaged from their bodies after each of the attacks, in the house mod I favored. Damn thing reached half as tall as my character, over time.

In all seriousness, the DB tried to get footing on Vvardenfell and were kicked back. Likely the only reason they are alive still on the mainland is because of Helseth. As long as he allows them to be there, they have a chance. So you know resources wasted are probably better then just being completely killed (which happens to them anyway, but they couldn't know that)

You're missing my point, because I stated it with a satirical angle. They are going to get killed off, just quicker by throwing resources away in utilizing the same sort of attacks always delivered under the same conditions. Generals have been cashiered in wartime for applying this logic--and for them, it might as well be a war. Someone at the top of their food chain needs a demotion.

Or a Fortify Intelligence Potion.

I was attacked by some Dark Bugger tonight, while I was enjoying a brief sojourn at Vivec's Guild of Mages. I rudely awakened by a fellow trying to kill me (I'm sorry but how good can the DB be if they send someone who can't even kill me when I'm asleep in front of him or her!?). I was much agreived by the impertinence, not to mention insolence, of this Daft Brother and I had little choice but to dispatch him immediately instead. What riles me most though is, whilst all this was going on, old Trebonius was swishing about like the Big I-Am, not giving me any help - nor did he even remark on the unfortunate situation unfolding within his branch of the guild! Well, as you might imagine, I was not pleased.

That's Trebonius for you. Maybe he thought the assassin was a side effect on one of your experiments of the topic Disappearance of the Dwarves. And why should he help you? Why should he be interested? He's better than that.

He needs to have a word with himself.

I doubt that he'd listen.

I've been attacked at several locations, like Fort Darius, the Sadrith Mora Fighter's Guild and the Balmora Temple. Nobody ever moves to help. A mudcrab or cliff racer wanders into town, and the guards wil chase it down and kill it. But an assassin from an illegal guild tries to murder me? Boethaiah forbid that somebody do something.

Crab legs are probably accounted a delicacy, whereas arm of assassin is a frowned upon entree in most cutural culinary establishments.

Once you kill him, just go to Mournhold and come straight back. Do not talk to anyone, not even the guy who wants you to perform in a play. You WILL get killed.

The play was fun. and you dont need to go to Mournhold to get them to stop attacking you.