Soul Reaper is a 
Raid Boss Lv. 2100 (260,500 HP) lurking in 
Haunted Castle, in the 
Third Sea. This reaper uses the 
Hallow Scythe's moveset and is summoned with the 
Hallow Essence.
How to get it
With the Hallow Essence in your inventory, stand on the altar with blue flames just next to the Haunted Castle's graveyard: the summoning triggers. Note: the boss doesn't appear at the altar, but in a castle room accessible through a hidden door.
What it drops
Bones- Title "Shinigami" (#172)
How to defeat it
It only has two moves, but with very short cooldowns: Death Cyclone, a tornado avoidable with 
Instinct that he unleashes when you stay at distance, and Soul Execution, a dash-grab that breaks Instinct and deals x1.4 damage if your health is below 50%. Indeed, below half health, switch to long range to prevent him from spamming Soul Execution. Big damage fruits (
Gravity, 
Dragon, 
Venom, 
Magma, 
Spider, Awakened 
Dough, 
Spirit, 
Buddha, 
Blizzard, 
T-Rex, 
Kitsune) work well; Buddha is one of the best choices despite its enlarged hitbox, and 
Angel (V3+) or Lei allow you to heal mid-combat.
Our take
An easy-to-summon reaper but dangerous in close combat: stay at distance, especially when your health drops below 50%.
Bosses
Orbitus
Order
Red Commander
Red Legionnaire
Rip Commander
Rip Legionnaire
rip_indra
rip_indra True Form
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Saber Expert
Smoke Admiral
Soul Reaper
Stone
Swan
Terrorshark
The Gorilla King
The Saw
Thunder God
Tide Keeper
Tyrant of the Skies
Unbound Werewolf
Vice Admiral
Warden
Wysper