Strange noticed this during their first fight inside of the Dark Dimension:

His attacks grew even stronger and Strange even questioned if there was a way to drain his strength. Later they showed Dormammu extent more energy to a level where the barrier used to hold the Mindless Ones awakened:

This means Dormmy was using the energy of his own dimension and the barrier stood even when Dormammu was killed in the past if you want to give credit about him needing to continue to use his powers to let it erected.

And if you're thinking I'm making assumptions, then it's just read what they show in the another comic:

Or in the one before this:

This is the reason for Dormammu becoming more powerful in the Dark Dimension. Anyway, it's just looking at his origin:

Another version:

The Dark Dimension wasn't even of Dormammu, in fact, it had a king and an entire population before him. It doesn't make sense for Dormmy to become more powerful just for stepping into another dimension, which wasn't even his and already existed before him. Most people have such ideas because of this panel:

But it never implied your logic of Dormammu becoming more powerful in the Dark Dimension and I maintain the oath being one of the explanations:

It's very clear that is the only reason that Dormammu gets weakened in other planes( Specifically the Earth-Plane):

Dormammu himself already said the he used o get weakened outside of domain:

And again:

I've read every appearance of Dormammuand the oath is the only thing you'll ever find about weakening him outside of his domain( Which include every other Handbook's page), so the reason for being in the Handbook of 2007 is probably a mention to his past weakness as the section was about his powers and there's an influence on them.

You didn't back up anything when your entire argument can only be supported by the Handbook's page of 2007 without any other further evidence or proofs. You didn't post anything aside from that page like scans confirming or other Handbook's to truly validate your claim.....It doesn't help you when your only evidence is the topic of the discussion and of my questioning.

Here's another evidence also proving my point of the Handbooks not helping at all ( Directly from Dormammu's Bio):

This creates a third option and most likely the true one.

It pretty much says that Dormammu hasn't any limitations( Such as getting weakened outside of the Dark Dimension as you're suggesting) and even go as far to mention his travels to other dimensions to back up even more my claim, so why the Handbook of 2007 is the only one who ever mentioned Dormammu abilities being restricted? Simple, it's making a direct reference to Dormammu's oath in the past for some reason.This is the only reasonable deduction as the oath is the only thing so far in all of Dormammu's comic history that makes him weakened and/or is showed in the very stories.

If you follow the Handbook's words blinded, then would mean that Dormammu suddenly gets weaker outside of his dimension......And I think you'll agree with me that this doesn't make sense without any explanations( The Handbook doesn't even give one). Dormammu never got weaker outside of his domain in all his 45 years, but the only exception was his promise/oath to Strange( Making the only option so far).

By the way....The Handbook sometimes contradicts itself and the one you posted is no exception.

It says that Dormammu's abilities are limited unless he can direct tap his energies via rebirth or/and symbiosis, but the last option only happened one time:

The problem is that Dormammu got much more powerful than normal, which would only be possible if he was at his full power before merging with his counterpart( I don't believe in this, but all my reasoning is following up your logic) and we can perceive this by Ancient One and Dormy's words. The entire thing gives the idea that Dormammu doesn't weaken outside of his domain if his power was already in his maximum before the symbiosis.

Not only that but the Handbook of 2007 usually let some information out, such as his appearances during the series of Darkhawk.

And you stop trying to act like you're not even a writer or a member of the company's staff. Dormammu appeared over dozens of comics since the oath was nullified and all even during all these different writers, Dormammu's past is mostly intact( Even having references to past events). The consistency in his history is present and even writers such as Steve Englehart( Who freezes the oath during multiple of his stories) never mentioned again.

Dormammu also died or was already dead when Strange sent his energies across multiple dimensions, in fact, we can see this by his dialogue with Stephen:

We even have Strange himself said that Dormammu needed them to reconstitute his essence( His life):

Another thing we have is Strange saying that his plan was to beat Dormammu before the same arrived at his full power and control, which according to your logic is only possible after being reborn on Earth:

The thing that most got me is that looks like you're not aware of said instances...Or the ones similar to that such as the scan posted below:

Dormammu's essence basically was regenerated on the Dark Dimension( Not even on Earth) by his followers and then put to ''grow'' inside of Earth's core....This is basically the same thing as gathering all his energies in multiple dimensions and the regenerated on Earth given that Dormy basically did the same thing in the past, in fact, I could say that it applies even more in the sense of ''Rebirth'':

Just look at the panel below( I even posted it before):

1- Dormammu used Gaea's energies to regenerate himself, just like when he used the energies of the heroes.

2- It reinforces, even more, the idea that Umar only brought Dormammu's essence to grow, which is similar to what the Mindless Ones did.

3- Dormammu's reconstituted himself....Like the other example too.

There's more:

Dormammu's energies were scattered all over the Cosmo Infinity( Just like the other example), thus it killed him as Dormammu himself says. This can lead us to conclude that Dormammu indeed died when Strange scattered his energies over multiple dimensions, in fact, the similarities are astonishing as Dormammu pointed out the needing to use energy to grow back( Reed's gun did exactly the same thing).

The Handbook pretty much says so and I know you love to use as an example:

So even if Dormammu being out of the Dark Dimension affected him somehow in the past( Which didn't), then the whole rebirth and symbiosis part to this not happen is done. Do you think that possess the right to speak about Dormammu's power level, friend? Thinking that the Dark Dimension amps his powers just shows what I already know...You shouldn't even have posted here before. After all...Why someone who lacks knowledgeable about the character in question should have a relevant onion? Another thing is that your username contradicts what you really are "Whoisthebest. You're far to being the best here, in fact, of all users in this thread, I would say you're the worst.