Interdimensional Olympic Bouldering
The last room of the cave is shaped like the inside of a geode, but one the size of a house. At first glance the crystal structure seems to resemble Nostronomium, so you spend a few minutes clambering around on the increasingly steep and convoluted crystal structure, high in the air. You've come to the conclusion that, however much it looks like it none of this stuff is actually Nostronomium, when back in the farthest recesses of the geode you find a shimmering, faintly-glowing pool of water, just a handful in a basin of crystal. Your talisman starts to glow blue in resonance with it, and--not daring to actually drink the water--you place your fingertip on the shimmering surface. That's all it takes, though, as with the chill jolt of landing in mountain pool the blue energy fills you.
The glow in the water fades, your talisman returns to normal, but you are forever changed. You gain 10 reflexes.