In order to get Apocalypse, you need to upgrade it several times by visiting the goblin NPC Switchstix, in Castle Zvahl Baileys.

Stage 1 will require:

Items:
Relic Scythe x1
Mammoth Tusk x1
Manticore Lth. x1
Rainbow Obi x1

Commission:
Lungo-Nango Jadeshell x5

Once you trade Switchstix these items, he will tell you to come back after one game day has passed.

'Relic Scythe' drops of Dynamis monsters, other items can be obtained through synthesis or trading certain items to Goblin Peddlers in Davoi, Beadeaux and Castle Oztroja.
Trade all items to Switchstix and he make your Memento Scythe .


Stage 2 will require:

Items:
Memento Scythe x1
Scythe x1
Bone Scythe x1
Death Scythe x1

Commission:
Montiont Silverpiece x16

Once you trade Switchstix these items, he will tell you to come back after a certain amount of time has passed.
This time he will take longer, about one week.


Stage 3 will require:

Items:
Dynamis Scythe
Attestation of Vigor

Commission:
6200 1 Byne Bill

"Attestation of Vigor" is required to change the 'Dynamis Scythe' to 'Bec de Faucon' which allows you to use the Weapon Skill "Catastrophe" in Dynamis only.
The transformation takes ONE EARTH WEEK and then the item is recieved back from NPC Switchstix.


Stage 4 will require:

Items:
Bec de Faucon
Tenebrous Fragment
Necropsyche

Commission:
10000 T. Whiteshell

Place:
North Gustaberg : Waterfall

"Tenedrous Fragment" obtained at the North Gustaberg Waterfall.
This will upgrade the Bec de Faucon to the final weapon Apocalypse , which allows you to use the weapon skill "Catastrophe" anywhere.