I've been thinking about this recently, let me know if it doesn't make sense.
Perhaps the Deceiver was once in the Heron Guard, but was kicked out, due to some problems he had (like his eye falling out, the Heron Guards love to look decent...). Or he may have gone past the traditional roles of a Heron Guard and been accepted by Connacht as an Avatar.
Proof? Well...first of all, Shiver says to him explicitly, "Will you bow to anyone who holds the throne of the Cath Bruig?" which all the Heron Guards follow, the Deceiver may have followed Balor because he thought him still the ruler of the Cath Bruig, once being Connacht. However, once Alric claimed the throne, the Deceiver decided to switch sides, to serve his emperor.
Also, the Deceiver talks about the "path to retribution," perhaps he feels guilty about the sacking of Muirthemne and decided it was time to help out.
The fact that nothing is known of how he has preserved his life for so long doesn't really add anything, but being part of the Heron Guard may explain why he has survived for so long.
(btw, after Muirthemne was sacked only 100 Heron Guards were left, the rest died in its defense)
