The mod changes some of the skills animations and effects that I felt were quite lacking in their original form. With the revelation that I could change animations as well, I've changed some animations with the old skill animations that I felt were cooler. I have also made revisions to the sound effects for skills that I felt lacked impact. This is a list of skills that were changed:

Hard Slash - HardDeliriumSlash
Syphon Strike - SyphonScourge
SoulEater (vfx)
Delirium - DarkDelirium
Bloodspiller (vfx)
Unleash (vfx)
Quietus (sound effect)
Edge of Shadow (vfx)
Flood of Shadow - FloodofShadowbringer
Shadowbringer - ShadowPassenger
Dark Mind - DarkShadowskin
Abyssal Drain (vfx)
Salted Earth (vfx)
Carve and Spit (vfx)

Any skills with a name change has a change of animations as well.

How to install (if new to the modding) with TexTools:
- download TexTools: https://www.ffxiv-textools.net
- install the tool and set it accordingly
- load the ttmp2 file and enjoy

How to install using XIVLauncher:
- download the alternative launcher XIVLauncher: https://github.com/goatcorp/FFXIVQuickLauncher
- install the program, and login to game
- click Dalumund Settings in System Menu
- add the following URL as a "Custom Plugin Repository" → https://raw.githubusercontent.com/xivdev/Penumbra/master/repo.json
- click "Save and Close"
- press ESC and click "Dalamud Plugins" again
- find "Penumbra" under "Avaliable Plugins" and hit install
- write /penumbra in chat, which will open a window
- import the mod and enable it

Things to note:

Maulclaw has mentioned that the animation swaps may not work 100% on lalafells, male roegadyn, and hrothgar. I have not tested the mod on these races so this may still be the case.
If you would like a more customizable old actions for DRK consider Maulclaw's mod: Classic Dark Knight Actions Restored