Death Mountain Descent
Death mountain descent is an Out of Bounds trick that allow to leave death mountain to reach the area bellow. Generally it is used to leave the Dark World part of Death Mountain because there is the only warp to the Dark World that doesn't need a Dark World item.
There are 2-3 different death mountain descents. They work the same way, but they don't lead in the same areas.
Dark World Cemetery/North of Palace of Darkness
It's the first DMD found in this game because you can do it without special trick, you can just use a the mirror. Obviously it's very slow. The point is to go in the south east part of DM and go through the border to reach the void. From this void area you can reach the rest of the dark world.
There are 3 main ways to clip in this edge, the easier and the fastest is to do a Spindash then hold >v near the corner to go through, you can use a+> too or the mirror, but it's definitely slower and more tedious.
Once you are in the void zone, you can go 2 ways.
and just fall down on the next screen
Behind Palace of Darkness
You have to go here, line up with the left of small "rock"
Now you are here and basically stuck without flippers.
But don't worry, now you need to fall left with <^ to load the right part of the screen (don't ask), then fall with ^> and hold > to do fakeflippers. After that you can just swim.
This one bring you close to the dark Kakariko town. Doing this one in the light world work too and you are close to the sanctuary.
Go at the bottom west on dark death mountain.
Fast Method (requires boots)
The fastest and easiest way to do this is to Spindash . Enter the spin dash state (superspeed not required) align yourself with the bottom wall and hold down/left. Once you jump down, start a dash, change your direction to up while charging, and bonk the upper wall. After the bonk, just hold down.
You can clip with <+a. If you don't have boots or you are a bunny you can try alternating <^ <v on one frame and try to reach one of this two vertical position once you clip enough into the edge to move diagonally. Y verticals position are 205 and 206.
The pink arrow try to show a small square of light pixel under link's shadow.
When you fall under the edge, DON'T go down imnediately, that will stuck you, move up first, then you can go down.
There are three ways to do DMD as bunny. The first involves a mirror transition. The fastest way is to do the Hera Clip, quickhop down off the ledge and hold down-right. You will nudge against a boulder. Align with this boulder (Link passes to the right of it) and move cardinally down until an obstruction stops you. You MUST have been aligned with the boulder for this to be successful. From there, move cardinally left into the warp. once the warp brings you to the Dark World, move cardinally left until an obstruction stops you. Then, move cardinally down until another obstruction stops you. Finally, press AND HOLD left. If you stop holding left, you will restart the walking animation, and have a 50/50 chance of softlocking. As you continue to hold left to walk, menu to mirror. (You may also menu to mirror at any point prior to this.) At this point, the easiest method is to use select buffers to get to the right pixel for the mirror transition.
Once you are on this pixel, cancel the pause menu and use the mirror on the right frame. If you do it correct, you will end up here:
From here, move straight left. You will end up on this screen.
From here, you need to make sure you correct the camera so when you transition into the screen above DW Kakariko, you do not softlock. Walk in alternating diagonals to do this easily.
Once completed, you will be in Dark World Kakariko as bunny.
The second method is slightly slower, but still faster than a manual clip. You perform all the above steps, but instead of mirroring on the exact correct pixel, you just walk into a transition. You will screen wrap to the dark world equivalent of outside the sanctuary. As soon as the transition completes, mirror to the light world. Dash left until you are aligned to where the sanctuary door would be, cancel dash right, and dash north into the sanctuary off camera. Immediately leave the sanctuary, dash south, and dash left. After the next screen transition, you will see your mirror portal right where you apparently left it.
The third method is a manual select/sword buffered clip.