This area will introduce another new mechanic in the game which is stone pillars you can move around. Just behind the waterfall is a pot that contains a sapphire. Push the block down to cross.
Move down to until the left turn. Stop here and collect the sapphire on the orange platform high above in the foreground.
Make your way to the side of the snake and kill it. Then move the two pillars towards the right-most wall. Climb up and grab the spear. While on this spot, grab the Frozen Spider fragment from the pot that is tucked between some tall stones that are near the snake.
Head over to the top of the pillars you moved. You can now throw a spear at it. Pull the left-most stone down once and the pillar on the right is pushed upward once. Cross the gap to the exit to the next area.