In Baahubali:The Beginning, During war, Kalakeya chief asks Sivagami, "You will declare King whoever kills me among both princes". This quote was said by Bijjaladeva a day before the war. My question is, how Kalakeya Chief knew about this? His spy Saket was arrested already.
It was never elaborated in the film but there can be two possibilities for it:
As mentioned by Tejashwini's comment, word travels fast when the news is that big. As it was a big deal for the kingdom and most probably words got around and reached to Kalakeya.
Kalakeya has another spy which was never revealed.