I found that title in the Monster Hunter game and it was translated as "A Meeting with Blazing Miasma", but what does せし mean there? It kind of sounds that it might be a case of classical Japanese grammar there, I think. I didn't find anything that could help me, so, can someone tell what it means? I think it must give some sort of nuance.
In modern Japanese, 邂逅 is a する-verb (which are also known as サ変), but in classical-esque Japanese the する becomes す (see 愛する vs 愛す etc). The し comes from the 連体形 (form used to connect to nouns such as 瘴炎) of the auxiliary particle き, which is a particle that is well known for having crazy conjugation patterns. き is used similarly to how た is used in modern Japanese to indicate things that happened in the past. Its supposedly unique point is that it's the past that one has experienced personally.
Now in classical Japanese the verb ending す's 連用形 (form used to connect to non-nouns) is し so you might expect the correct conjugation to be 邂逅しし. However, き with all of its odd conjugations has a special rule where if it connects in the 連体形 to a サ変 verb, the サ変 verb instead connects in the 未然形, which is せ. That is why it is used as せし
Here is the progression:
I got the image from this page about き and the related けり (the past that one knows about through hearsay).