Who gets it: Achaean League

"In the early summer of the year in which Agetas was strategos of the Aitolians (217 BC) [...] Aratos (of Sikyon) had found the mercenary forces of the Achaians disaffected and the cities not at all disposed to tax themselves for the purpose of maintaining them [...] (5) However, he made an appeal to the Achaians, and obtaining a decree on the subject, occupied himself actively with preparations for war. (6) The substance of the decree was as follows. They were to keep up a mercenary force of eight thousand foot and five hundred horse and a picked (epilektoi) Achaian force of three thousand foot and three hundred horse, (7) including five hundred foot and fifty horse from Megalopolis, all brazen-shielded, and an equal number of Argives." (Polybios of Megalopolis, Histories 5.91.1-8)

It fires when at least 25 battles have been fought by Achaean League.

The game checks this at the end of every round.

4 units can be recruited once it fires:

Unit	Who can raise it
Achaian General's Bodyguard (Reformed)	Achaean League
Achaian Thorakitai	Achaean League
Argive Epilektoi	Achaean League
Megalopolitan Chalkaspides	Achaean League

Units already in your armies are converted:

Before	After
Achaian General's Bodyguard	Achaian General's Bodyguard (Reformed)

1 unit can no longer be recruited:

Unit	Who can raise it
Achaian General's Bodyguard	Achaean League