Will I lose my player if I play the DLC, or can I use him in the expansion?

I have the uncapper mod and I am currently level 142 or something like that, it's a lot of invested time and I am quite proud of my 340 odd one handed damage from an Ebony Sword

You can use him in the expansion. Not sure why so many people think that Dawnguard is seperate from the game.

Because it sounds like it's very big... and considering some games -Dragon Age- do most there big expansions as stand alone DLC not as add-ons/expansions

-edit- But yes you can use your already existing character for the DLC, Dawnguard only expands skyrim and add two or more large areas, with a main quest arc and some side quests... and ONLY that

If you use any mods, check if they are updated for any expansions/ fix updates. If you have the updates and dlc with older versions of some mods, you could have serious issues.

Bethesda ( in terms of TES) does add-on expansions not seperate 'missions' ala gears 3 for example.

Okay, thats reassuring then. Thanks folks.

Just back up your saves. Level 142 would be a pain to lose.

Been noticing that.
The only games I know with DLC where "carrying over" or "playing as someone else" was a thing has been Dragon Age: Origins and Grand Theft Auto 4. Most everywhere else, DLC is integrated into the core game.

I has a question , is this going to be like shivering isles in the way that you HAVE to have the MQ done to start it ?

...You didn't have to finish the MQ to start Shivering Isles...
The Oblivion Crisis isn't even mentioned.

There is absolutely no way to predict whether or not a mod will conflict with an expansion or even a patch, for that matter. To be on the safe side, backup your saves and do not load this character in Dawnguard until the author updates his mod.