The text is:
ntmilEmondt enae Leala ed hThtrc smve till pg n tut ane una olas hrertwil incma t ieeenak ioo anTl es otah foer del smeormresr nmhi m f ugrpsmci io thmmoT tir luay beoa a nfsoner t oris sihhsvlad sb fengtfned ewr irnssortaa htam ervegs hntgdn i senly ia eoftn bito itofshirude trsecolucfia ra phef fid tyaene thowuoyffi w ro nde sr pnfct ta T f et e a lir fhiout ioilimos utortru tn c t nce raero ls n osenn bo Ee ml Elt stigsimtet
It comes with the following riddle, which is part of the intended form of solving the puzzle.
Where Air meets fire
and land marries sea
head south and east
Therein lies the key
I'm pretty sure it's a transposition cypher, based on letter frequencies, but beyond that, I have no idea.
I know that the end result plaintext is a spell in 5th edition D&D, which means it's (probably) in the format:
[Ordinal] level [spell school]
The [spell school] is Transmutation, and I know that the spell level is a high one, so the [Ordinal] is probably "Sixth" or higher ("Ninth" being the highest possible). This was learnt from in character knowledge. There's also a well above zero chance that the first word of the [Title] is "Elminster's" or "Elminster".
Does anyone know any good analytical techniques for solving this sort of transposition cypher, some possible cyphers it could be, or perhaps , way of solving it through the "intended" method using the riddle?
Someone asked a question as an answer, and then deleted it (obviously, using an answer to ask a clarifying question is not the way to go about things). The question was something along the lines of 'is it a spell from the D&D 5e PHB?'
The answer was no, probably not. It's a spell for 5e, but this is the third of three puzzles, and the first two were homebrew spells, so I suspect the third will be too. The previous two's names both started with "Elminster's", which is why I suspected this one might as well.
I used some in-character knowledge to get a clue. Apparently "fire" refers to heating the scroll, and "air" refers to the blank spaces in the text. When the physical scroll was heated* the following map appeared. The blue dots weren't actually there, I've just added them to make clear where the blank spaces are. For reference, the map is of the Moonshae Isles, in Faerûn (part of the Forgotten Realms D&D campaign setting).
Apologies for for the fact that some information was required that would have been impossible for anyone here to obtain on their own. It never occurred to me that such information would have been part of the puzzle.
* unfortunately, the invisible ink had faded by the time I tried this, so it didn't work. The above image is of a separate copy of the scroll, with the text superimposed over it digitally.