My Shadow

The first steps should be trying to find a new mob "Lost Soul", these dark skeletons rarely spawn in groups at night. Their drops aren't that important, though by checking the achievments tab after slaying one will give a hint to the player about where to go.

Go to the center of the world (x = 0, z = 0) and dig straight down until you fall right into an abandoned underground study, there you will face some monsters and monolith that will constantly give you debuffs. Be sure to destroy it! On the lowest floor alongside a ruined portal you'll find a skeleton. Break it by right clicking it to grab the Cursed Relic stuck in its chest. Be sure to collect the 3 pages scattered across the tower and craft the Tome of Shadows - It will give you valuable information and hints about the curse.

Right click the relic to trigger the curse. The effect will not go away on its own, instead an invisible timer will start counting up. The timer will only be active at NIGHT in the DARK (so no torches!) directly UNDER THE SKY. Only then will the curse be active and start triggering different events. The curse has 3 stages, each taking about 20 minutes to complete. Depending on the stage, the curse effects will start triggering more often, sometimes even two at a time! Dying with the curse resets the progress.

Once you complete the curse, your relic will transform. Once you're ready, drop it on the ground from your inventory to summon the Shade. Be wary, the Shade demands a fair duel, so you won't be able to place any blocks! Avoid the Shade's blade and good luck!

After defeating the Shade your relic will change once again. This time right clicking with it will open a window in which you'll be able to select a shadow minion. Deploy them by right clicking a block while sneaking. The shadow minions are invincible but will disappear after time. You can recall them early by right clicking them with the relic. Congratulations on beating the mod!
... or not?

Right click a beacon while sneaking with the relic in hand. You will banish the spirits and empty the relic. Now follow the Tome of Shadows' instuctions and craft the Reaper using the empty relic and the 4 different parts. A recipe should appear in your recipe book. Craft the weapon, press V while holding it in hand and enjoy collecting souls!