I've completed all 10 Dawnguard main quests, including killing Harkon, and made several entrances into the Soul Cairn but have yet to run into the dragon Durnehviir. Where can I find him to be able to get the Summon Durnehviir shout so I can unlock the Soul Tear?

How did you manage to finish the Dawnguard main quest without fighting Durnehviir? You should have met him again when you left the Boneyard, he would have given you all three parts of the summon shout then.

exactly you should have met him while your fighting him or did you fight him then run off lol

You SHOULD already have it.

Sorry? You can't complete DG without fighting Durnehviir! Fighting him gives you the second Elder Scroll.After you fight him, he's waiting for you outside the Boneyard and the game forces you into conversation with him!!

what everyone else said. If you ran into him and didn't realize what happened (I dunno talked to him but ran away?) then you may have issue, or it may have given you the summon anyways.

After you learn the summon you gotta summon him 3 times to learn the shout

Would not surprise me to find out it is a PC player who just skipped that bit using console commands, but I am all ears to hear how it went wrong some other way.

First, I did not use any console cheats!! Second, I spoke to Serana's mother several times, including the last where she's going to return to the vampire castle now that Harkon is dead, but she never mentioned the dragon in any dialog that I remember, killed the guardians, entered the boneyard and got all the stuff there including the scrolls, left and returned to the Soul Cairn multiple times, all without seeing or hearing about Durnehviir. So if I missed the talk about the dragon, wouldn't he still be an objective whenever I enter the Soul Cairn? I've even entered the Boneyard several times to see if I could set a trigger, but nothing.

your game is bugged then because before getting the scroll from serana's mother you have to fight the dragon...

OK, I noticed in one of the you-tube videos for the Soul Cairn playthrough that Serana stops you from entering the Soul Cairn right at the beginning, offering to soul trap you to help enter. That did not happen to me - I just entered. So from the previous reply, before I can enter the Boneyard (or right after I enter) I have to fight the dragon? My DLC must be bugged because I'm at level 48 and I've played entering the Soul Cairn twice (once from a previous savegame) and I don't ever find the dragon. Man, I hate to do this all over again because I've gone on to complete the Restoration of the Thieves Guild, and am ready to side with either the Imperials or Storm Cloaks.

Anyway, thanks everybody for your replies!

He should show up twice when you first enter the soulcairn.

if you are already a vampire, serana will NOT ask you if you want to be soul trapped.

But you are a PC player? Do you have any mods that would prevent a dragon appearing? Did you somehow start Skyrim by skipping something important to the dragon theme of the game? Do you get dragons in the main game? Even so Serana's mother should have mentioned the dragon, and I do not see how a bug would change dialogue like that. And as already mentioned, if you are a vampire you do not need to be soul trapped to enter the Soul Cairn.

Did you start the main quest line which unlocks dragons?

I played DG as a vampire, not sure if non-vampires can get that summon? Maybe you had the regular vampire disease but didn't know it at the time of entry and it botched up?

If you do Dawnguard before the MQ, the dragon won't be there?

You do not need to do the main quest, although you might have to start it at least, if you want dragons. One of my characters went through Dawnguard and she has only went as far as visiting the grey beards in the main quest, she has no self spawning dragons, Alduin has not woken them yet. But I thought just escaping from Helgin would have been enough, maybe you need to go as far as Dragon Rising to get dragons.

You have to escape Helgen, which there is no way around that, get to Riverwood and then go to Whiterun, talk to the Jarl and his mage, then go to the western tower to aid in a dragon attack. That is the point where the game unlocks dragons and now random spawns them. From what I recall reading in another thread, it is required in order to do dawnguard spawned dragons, such as the one in the valley where you take the journey of the snow elves.

Don't know if that applies to Durnehviir but if nothing is showing up, it's a big possibility.

I believe you can meet up with Durnehviir without becoming Dragonborn. I posted a thread asking this question and got replies saying it was so.

Really? I only met him once and that was the battle in the Boneyard. Perhaps he is radiant then becomes scripted when you enter the Boneyard, though idk how that would work.

Twice I believe.
(1) when he has reacted to the collapse of the barrier and attacked and been killed.
(2) after returning from the Boneyard he is perched on a structure and again confronts you.

On both occasions he is unavoidable. As soon as you exit the Boneyard you are in conversation with him. You chat. He asks a favour. He gives the full summon shout. You are pretty much given the shout whether you want it or not.

Yup, it's true--I did all of Dawnguard using the Alternate Start mod where you never even flee Alduin at Helgen, and I still saw Durnehviir. There's even some extra dialogue if you don't yet know you're the Dragonborn.

As others have said, there is no way to get the elder scroll without triggering the dragon fight. And there is no way to leave the castle without triggering the dialog to get his name.
After your two encounters with Durnehviir, he hangs out in a small building to the left of the castle with the two flaming turrets (where you meet Serana's mom.) He perches on a broken word wall and there is a chest with some goodies next to it.

This is actually not necessary at all.

I went straight from Helgen to leveling up to lvl 10, and went straight for the dawnguard quest line.

I finished the whole thing before initiating the dragon attacks.

You have to be level 10?

it's the recommended level for starting dawngaurd, and is when guards talk about them giving the side quest to start the dawnguard questline.

However if you know where you're going it can be started at any time you want, even as soon as you get out of Helgen. But it'd be a bit weird to do that since you'd have a lack of weapons armor and experience.