We know that he is powerful enough to travel cross realms, realities and to other worlds; in order to complete his research, but just how powerful is he?

It seems to me that to complete his research, he needs to go to places and encounter beings that are not only dangerous but out right hostile to life in general.

So how has he survived to write such epic books?

PS:

Obviously I am talking about him in terms of the D&D universe.  To settle an argument before it starts, I know he's not real LOL.

That's interesting because when the weave was re-established, a rule was built into it that prevents any mortal creature from casting anything more than a level 9 spell.

So if wizards like Mordenkainen are able to acquire the ability to cast 10, 11 or higher level spells - then that kind of says that it's not impossible, just extreemly difficult.

Perhaps the difficulty is in the acquisition of the prerequisites knowledge.

Something normal wizards are not able to obtain, but which non-the-less remains obtainable with enough time and study.

Since Mordenkainen is essentially the one of, if not the, greatest of sages (don't think he's actually a sage - just saying) it would make sense that he possesses the knowledge to and by extension ability to weild such powerful magic.

If this is indeed the case, and the ability to weild higher level magic, is not specifically blocked but rather requires an extended ammount research and practice, then that would say other forms of wizards (liches or other undead or otherwise immortal casters) could also acquire the ability to cast 10, 11 level spells.

That would be an interesting path for a wizard to take I think and I wonder what one would do with that much power.  

After all, we have already seen how someone with an enermous amount of power, caused the downfall of an entire civilisation and almost destroyed magic all together.

And then there's the old saying "power corrupts and absolute power corrupts absolutely"

I wonder if that would be a really hold true.

Quote from Conn_Eremon >>

I think it's been stated by folks like Perkins in the podcasts that wizards like Mordenkainen break the mold, attain power otherwise unattainable to other wizards. In 5e terms, they're casting 11th level spells, essentially.

Exactly how powerful, I don't know.

He's not that strong. He appears in a published adventure with the statistics of an archmage, albeit with a personalized spell list.

In a v3.5 adventure he appeared in, Mordenkainen was level 30. In his most recent appearance, as noted above, he's "just" an archmage.

That's the magic of an NPC - he can be exactly as powerful as is needed for his role in the story of whatever campaign he pops up in.

One thing that I can address though is this: "...when the weave was re-established, a rule was built into it that prevents any mortal creature from casting anything more than a level 9 spell."

Mordenkainen is a native of Oerth (Greyhawk) not Toril (Forgotten Realms), so the weave (a Forgotten Realms setting detail) has never really applied to him.

that's actually something that I never considered

Quote from AaronOfBarbaria >>

 

Mordenkainen is a native of Oerth (Greyhawk) not Toril (Forgotten Realms), so the weave (a Forgotten Realms setting detail) has never really applied to him.

It's probably a retcon but in 5th edition all magic in the multiverse relies on the Weave, as explained in the Spellcasting chapter of the Player's Handbook. "The Weave" is just what it's called in the Forgotten Realms, but it's still a thing in other worlds.

Mordenkainen was a Gygax creation and is not a part of the Forgotten Realms setting. However if you consider the D&D multiverse, since (nearly) all settings are connected in some way by Planescape, Ravenloft and Spelljammer, you could probably find some comparisons. Purely comparing him to Forgotten Realms wizards, I'd probably place it in this kind of order, from weakest to strongest.

Elminster < Mordenkainen < Karsus

I know this is several months ago, but, I thought I would add a few interesting notes to the topic. 
…
While there aren't the same kind of statistics published for Mordenkainen in 5e as there have been in previous editions, some of the interesting lore still applies, I think.

In previous editions of the game, Elminster, for example, was technically  higher level than Mordenkainen (according to the statistics in the 'Epic Level Handbook').  But, the catch was that he had Prestige Classes like "Chosen of Mystra", in addition to levels in multiple classes like fighter, rogue, and cleric, etc.  Mordenkainen, however, was a straight-class spellcaster, specifically a Wizard.  So, Elminster, for example, might've technically been a more "powerful" character, since he had a higher CR and more levels … but, Mordenkainen was the more powerful wizard, which is the OP's question. 

Mordenkainen had the feats which allowed for "normal" spells to be cast at functionally higher levels to allow for things like Maximized, Quickened, or Heightened 9th level spells which use higher level "slots".  He could use metamagic to raise functional spell levels up to 12th level, while Elminster, for example, could only do 10th. 

Which leads to the discussion, above, regarding higher level magic.  9th level spells have always been the "cap" for spellcasters and it's always been said this is the most powerful magic there is, short of gods' magic, there's nothing else higher than 9th level.  I mean, 9th level spells can reshape time and reality, so, one doesn't get much bigger than that.  … Unless and until that edition has a Deities and Demigods supplement, or Epic Level Handbook supplement, or something like that.  Epic Spells can do things like create whole new planes (or demiplanes) and create whole new true-breeding species, so, that's a whole other kind of magic. Epic Spells are technically higher than even 12th level spells.  

Back to Mordenkainen, then;  in addition to 12th level spells, he could also cast those higher "Epic Level" spells.  Elminster (by that book's statistics) couldn't cast Epic Level Spells.  

So, it's quite likely that Mordenkainen is the strongest mortal wizard  in the Prime Material plane.  There could in theory, be a stronger spellcaster out there in the multiverse  … but, as far as we know, Mordenkainen is the only one able to do the seriously  top-end kind of stuff.  

Quote from IcarusATB >>

I know this is several months ago, but, I thought I would add a few interesting notes to the topic. 
…
While there aren't the same kind of statistics published for Mordenkainen in 5e as there have been in previous editions, some of the interesting lore still applies, I think.

In previous editions of the game, Elminster, for example, was technically  higher level than Mordenkainen (according to the statistics in the 'Epic Level Handbook').  But, the catch was that he had Prestige Classes like "Chosen of Mystra", in addition to levels in multiple classes like fighter, rogue, and cleric, etc.  Mordenkainen, however, was a straight-class spellcaster, specifically a Wizard.  So, Elminster, for example, might've technically been a more "powerful" character, since he had a higher CR and more levels … but, Mordenkainen was the more powerful wizard, which is the OP's question. 

Mordenkainen had the feats which allowed for "normal" spells to be cast at functionally higher levels to allow for things like Maximized, Quickened, or Heightened 9th level spells which use higher level "slots".  He could use metamagic to raise functional spell levels up to 12th level, while Elminster, for example, could only do 10th. 

Which leads to the discussion, above, regarding higher level magic.  9th level spells have always been the "cap" for spellcasters and it's always been said this is the most powerful magic there is, short of gods' magic, there's nothing else higher than 9th level.  I mean, 9th level spells can reshape time and reality, so, one doesn't get much bigger than that.  … Unless and until that edition has a Deities and Demigods supplement, or Epic Level Handbook supplement, or something like that.  Epic Spells can do things like create whole new planes (or demiplanes) and create whole new true-breeding species, so, that's a whole other kind of magic. Epic Spells are technically higher than even 12th level spells.  

Back to Mordenkainen, then;  in addition to 12th level spells, he could also cast those higher "Epic Level" spells.  Elminster (by that book's statistics) couldn't cast Epic Level Spells.  

So, it's quite likely that Mordenkainen is the strongest mortal wizard  in the Prime Material plane.  There could in theory, be a stronger spellcaster out there in the multiverse  … but, as far as we know, Mordenkainen is the only one able to do the seriously  top-end kind of stuff.  

So this is how he was able to create spells that let him do things like create his "magnificent mansion" and other things.

Based upon this, I would stick my neck out and say that maybe Mordenkainen wasn't entirely mortal, that he was some kind of demi power and probably bordering on lower level god.

Though these days, now he has lost his mind, he is probably a lot weaker, but much more dangerous than he used to be.

Quote from SocialFoxes >>

Quote from IcarusATB >>

… a bunch of stuff …

 So this is how he was able to create spells that let him do things like create his "magnificent mansion" and other things.
Based upon this, I would stick my neck out and say that maybe Mordenkainen wasn't entirely mortal, that he was some kind of demi power and probably bordering on lower level god.
Though these days, now he has lost his mind, he is probably a lot weaker, but much more dangerous than he used to be.

 Well, I don't know whether he was "entirely mortal" or not … 
But, in Greyhawk lore, he is known to have extended his youth, at the very least.  He is much older than he appears.
I believe that it's said somewhere that he may have used Potions of Longevity (in older editions).

Karsus and Ioulaum are both level 41 Arcanists as of 2nd edition. Kept Ioulaum the same level but turned 10 of those levels into prestige spellcaster classes. Elminster as a spellcaster only has around 26 or so levels. Larloch is in the early 30's. In 3rd edition, Mordenkainen has 27 wizard levels and in 5th he's only level 18.

Karsus is the absolute king of all casters in the D&D universe that I can find, especially so since Faerun became the default setting. The guy is responsible single-handedly, for the fall of what equates to 'the great ancient empire' in the Realms.

Quote from golw >>

Karsus and Ioulaum are both level 41 Arcanists as of 2nd edition. Kept Ioulaum the same level but turned 10 of those levels into prestige spellcaster classes. Elminster as a spellcaster only has around 26 or so levels. Larloch is in the early 30's. In 3rd edition, Mordenkainen has 27 wizard levels and in 5th he's only level 18.

Karsus is the absolute king of all casters in the D&D universe that I can find, especially so since Faerun became the default setting. The guy is responsible single-handedly, for the fall of what equates to 'the great ancient empire' in the Realms.

Karsus is the only mortal to ever be able to cast 12th level magic, and in the process became a God, at least for a short time.

However, despite becoming a God, Karsus was not Divine and couldn't control what he had created or became, which ultimately is why Mystra killed herself.  As a God, she would be reborn, but killing herself would also kill Karsus, allowing the newly reborn Mystra to bring magic back into balance.

I see her her actions as being her, sacrificing herself, to prevent magic from destroying the world and honestly believe that if she had of been able, she would have saved Nethiril as well.

The fact that Karsus was able to do what he did, and the the outcome of what he did, combined with the danger he represented, makes him the undisputed King of all mortal spellcasters.  

Nobody else could have achieved such greatness,or caued so much devastation with a single spell, and now nobody ever will.

Mordy is a plot device.  At this point, he has no stats; all his spells are DM hand-waving.

Quote from FoxfireInferno >>

Mordy is a plot device.  At this point, he has no stats; all his spells are DM hand-waving.

I had thought about running a campaign once, that was basically about the daughter of Mordenkainen, who hired a group of adventurers to protect her as she hopped through the various planes, looking for her father.

So the party would get to visit all the places, and see all the monsters and NPCs in Modenkainen's Mordenkainen's Tome of Foes.

Sadly that campaign never came off.

This is interesting. I might include acquirable traits or feats so that my players can go to 10th level spells post 20th level. I think the spells would act like this:there would be higher level cantrips (like being able to nonchalantly teleport long distances as a cantrip) and then there would be higher level spells that operate off of a spell slot system (the reason for this is that i imagine powerful wizards like mordenkainen to be able to easily transport long distances or conjure flames or things like that). If you think its a good idea feel free to steal it, im okay with it.

I'd say that Mordy's published stats at any time are basically how much power he feels like utilizing at the time.