DM help!

[SMac8988]

Hey all. I am running my group tomorrow and am stuck working a 24 at the fire department away from my books.

What I need help with is I had built my entire next session around a water boss fight in the underdark, planning an aboleth as the boss, but have decided against it, wanting to use the creature as a later encounter.

I was wondering if anyone knows of, or can help me fin some type of amphibious cr 8ish mob that would have like a following or creatures I could control.

The fight is suppose to have two outcomes the group will be at a dam that a small city of mushroom people will be helping them. Around the finial fight they will have the chance to open the dam to make the fight easier, but at the cost of all their new friends lives, or have to fight a terrible beast in the water.

The story line is meant to continue through an underwater tunnel in the flooded city the dam holds underwater. So it can be a larger beast that lives within the sunken ruins.

Thanks soooo much all!!!

[Giant2005]

How firm are you on the CR 5 thing? Sahuagin sound like they would make good candidates for what you want, but the strongest of them is CR 5. They can also telepathically control sharks, so that could make for a good giant monster for you, but the Giant Shark (a huge beast) is also only CR 5.

[SMac8988]

That could work well. Have the final fight be two giant sharks and one of the big guys.

And I'm not dead set on it, just about where the group is. There is 4 to 6 level 5s so, the boss fight should be a bear deadly challenge for them.

[Drackolus]

It's the underdark. Make it a sentient psionic shark/octopus creature with sahuagin thralls.
EDIT: just remembered that sharktopus is a thing. Even better.

[SMac8988]

It's the underdark. Make it a sentient psionic shark/octopus creature with sahuagin thralls.
EDIT: just remembered that sharktopus is a thing. Even better.

The idea was that the group would have to fight their way up the dam, turning it into a dungeon crawl, fighting a few sub-bosses and a few puzzels, then the big fight would be started with then over looking the sunken city atop the dam. And give them the option to weaken the fight by opening the reservoir, then draining a large portion of water, but drowning all the mushroom people who will provide them shelter from attacking cave beasts.

As is in the way down they will run into umber hulks and stuff. So the fight can be very difficult, my group is normally pretty resourceful.

I think an octopus wouldn't really work as a boss figure.