Converted Lichs should be Immune to Radiation as they were before conversion

[(XBOX)LadyWinterstorm]

I just had a Converted Lich show up randomly during a Sortie Radiaton Hazard mission. 

Lichs as enemies are immune to the normal Radiation effect, as allies they should also be immune, let them take more damage from allies in the same way they do as enemies 

[Drasiel]

They're not really immune to radiation when they are enemies. Radiation does work differently on them though, it causes the other enemies near the lich to attack it but the lich will still focus on you and not the enemies.

If you wanted player owned liches to work the same way they should continue to attack enemies but if you, your pet, or your teammates damage the lich it hurts it.