The song is Stage35 by Kevvviiinnn.
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Since this got bumped, I might as well give an update. IX-Pack v1f is going to be released at the same time as MM8BDMv3a's release. There isn't anything major like new maps, as I've been busy with mapping and managing the MM8 expansion. But this is the current changelog for major changes.
IX-Pack v1F: MM8BDMv3a Obligatory Update
-In General - Most maps have been updated with MM8 stuff.
-IDM02 - Updated Layout + New song by Mr. X
-IDM05 - Oil Pit replaced with a new area, upper area revised slightly.
-IDM06 - Floor of Shame is gone.
-IDM08 - Fixed Spawn.
-IDM09 - Cossack Base and Freeze Man Cave redone.
-IDM11 - (Finally) Redid that DAMN (not fourth) map. So long my most hated level in IX.
-IDM12 - Mowed the Lawn, added Thunder Claw Pegs.
-IDM13 - New version of the song by Llamahombre!~
-IDM20 - Finally ensured all the block player lines are placed.
-IDM21 - Fixed hiding place + removed a few of the trees to cut down on some lag that it generates
-IDM22 - Oil pits use the new MM8BDMv3a method, so any fire weapon can set them off.
So as usual, now is a good time to throw suggestions at me.