• This module is 3/4s combat, with half of it being a three-stage boss fight. Needless to say, this review assumes that you're running it for players who all enjoy the occasional heavy combat module.
• All in all, the combats are decently thought out. They are certainly much better designed than the average module, yet for the sake of making DM adjustments, I will list my nitpicks below (vague spoilers!).
• The module does not include the two "global" effects which happen during the red war. Although there is a random "war events" table to be used during the one above-ground encounter, it's not quite the same, this may be a disappointment to some of you who enjoy chaos. Nevertheless, this is a pretty intense module so adding more things to keep track of may not be wise.
• Stage 1 of the boss fight is simply not threatening. I had the side-monsters try to grapple enemies into lava, and I had the fire-immune boss himself simply wade into lava at one point to throw a wrench into the enemy plans. Even then, the fight seemed to have too little damage but too much hit points. The legendary actions are a little too boring.
• Stage 2 of the boss fight is where your party may start to find problems. My party had spell selection and mobility/ranged problems, so they were already starting to find problems with this boss who had suddenly become more mobile. Were this the end of it, I think it would be sufficient to say that this is simply a learning experience for the players that they need to pack some ranged capability.
• I completely missed that the magic item in the second stage of the boss fight has two abilities (the module reminds you of one, not that they are to blame) and let him fall off the mount, this is a horribly awkward waste of actions that probably defused the tension. Mea culpa.
• Stage 3 of the boss fight is an incredibly over-CR'd monster that the author seems to have balanced by providing several NPC-buffs. What it doesn't quite adjust for is just how ridiculously mobile stage 3 is. Some parties will have no trouble with this, but it's quite possible your party will. There's really nothing stopping the enemy from damaging players without really giving the average much chance to damage it.
• It is completely probably for the DM to play the monster "stupidly" by occasionally ending in range of the party's attacks if this fight is to be balanced.

If I were to do it again, I would shorten stage 1 and lengthen stage 2 somehow.