Yup, I was wondering about having the burning zombies replenished by returning to the wall and maybe having a 'burn out' time outside the wall. That would limit their numbers to the wall's engulf capacity.
Sounds like we are on the same page here. I'm not sure if everyone will agreee, but to me it makes more sense for Burning Zombies to be an extension of the Carnivorous Wall. (In a way, they remind me of the Shivak creatures that serve The Spelljammer.)
The original module has burning zombies wandering about the labyrinth and makes no mention of them ever returning to the walls, which would imply that they can last a long time outside the wall that created them. I do like the image of a wall attacking by vomiting pseudopods and flaming corpses, so I'd like them to be stored/refueled by climbing back into the wall.
You know, looking at the Burining Zombies I have to wonder what the point is. Why would the wall go to all the effort of creating zombie-like creatures that wander off and don't provide it with any benifit.
I think that the Burning Zombies should have a Shivak-like function and should aid the wall in some way. Considering that the Carnivorous Wall is fairly immobile the Burning Zombies could help it eat.
How about they pick up dead, dying or unconcious humanoids, take them back to their walls and then drag them inside? Behavior like this would sell them as being in a symbiotic relationship with the wall. And if a PC's cohort got knocked out and grabbed, this sort of thing could provide some interesting tension in an encounter.
It might even be fun to have large Burning Zombies grapple smaller opponants and try to manhandle them back to their wall while they were still alive and kicking.
Although Carnivorous Walls have an "immune to fire" Special Defence in the original stats, burning zombies do not. There is also no mention of them having the sluggishness of standard zombies (they don't always lose initiative like common zombies did in 1st/2nd edition AD&D). Hence, I did not propose to give them fire immunity or the single action quality.
However, it does say the flaming liquid does no damage to the zombie - so I think the best solution is to give them immunity to the flaming liquid of carnivorous walls and burning zombies, rather than a general immunity/resistance to fire.
I've just realised the blurb says their heads are coated with the liquid but only their bite causes fire damage. That doesn't seem logical to me. Why can't they do punching damage with fists of fire? Why can't they headbutt for flaming damage?
Maybe it would be more appropriate to put the fire
inside the creatures and have flames pouring out of their mouth when they open it.