Thank you again! Fiddling around with the custom effects worked out quite well.
Here's what I came up with:
(CODE) Festering Aura; AURA: 15 cube,once,cloudkill; AoE Festering Aura; IF: FACTION(notself); IF: !CUSTOM(Undead Tamer); SAVES: (SDC) CON; SAVEADD: Festering (/CODE)
And the custom effect:
(CODE) Festering; poisoned - one turn, Remove Start Turn (/CODE)
One final thing (not related to FG proper); how do you list the code in the scrolling tab.

