I think Tier4 modules are hard to write. This one is great.
Story : excellent, great follow-up of the Baron Rajiram story that started in T3 modules.
Difficulty : Easy. DO NOT FORGET THE UNDERWATER COMBAT RULES. But my party had 3 casters out of 5 so they were not penalized by the underwater. If you have more of melee weapon and range weapon fighter, the fights will be harder.
The last fight is a nice challenge for the DM. You have a mix of spellcasters and non-spellcasters to control, there are like 5-6 different stat blocks (at least only one spellcaster type). Be ready and try to memorize most if the stat blocks or else you will kill the momentum.
The multiple psychic damage trap were really cool too! You can DM this in a way that can be really confusing (but fun!!) for the players.
Also, no whining over magic items... like Oprah would say : ''you get a magic item! you get a magic item! EVERYONE GET A MAGIC ITEM!''
Just beware, there are many options from which the player can choose and my players took like 10 minutes to choose. This can be quite annoying in a limited-time setting.
[4 of 5 Stars!]