From Destinypedia, the Destiny wiki
Milos was gifted to Caiatl at a very young age from her distant father, Calus. Caiatl grew extremely attached to her newfound companion, spending every waking and dormant hour with Milos by her side. Caiatl trained Milos herself and even fed him from her own plate. Calus grew jealous and ordered one of Caiatl's servants to butcher the War Beast to punish Caiatl for loving something or someone more than her father.
Despite Milos' death happening so long ago, Caiatl still remembers his presence every time she closes her eyes. His death was also the main motivator she had, along with seeing Calus was weak, for Caiatl to participate in the Midnight Coup. Even to this day Caiatl still holds malice towards her father for Milos' death and swears revenge will come by her hand one day.
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