I forget what it is called and where it is located but i think you need all the dragon keys to unlock it.
I found a note that said something about ancient evil blah blah blah derp derp derp.
If you know, what is this "ancient evil"?
Sorry its by a waterfall.

You have to find all of the Gauldurson brothers (there are three of them) in dungeons throughout Skyrim. Did the note say something like "Be bound here [insert name]"
or was it something about an ancient evil being erased from history?

with a dead adventurer next to a podium type thing

Yeah, that'd be the dungeon that you need all the amulet fragments in order to claim your reward.

You'll need to complete the quest Forbidden Legend, which is activated by finding an amulet fragment on the corpse of one of the Gauldurson brothers. Explore the dungeons of Saarthal, Folgunthur, or Geirmund's Hall to find an amulet fragment and start the quest. Or start the quest by reading the book "Lost Legends" which can be found in Dragonsreach next to the Jarl's room.

Once you start the quest just follow the objectives and you'll eventually find yourself back at that dungeon behind the waterfall where you'll claim a reward.

thanks

Your welcome, but also keep in mind that the first quest for the College of Winterhold ties into the exploration of Saarthal, so you might want to start that quest before entering Saarthal.

i already did that