Quote from SnargulIt's not the Lich King's buff stacks that need to be tracked but the Necrotic Plague itself that jumps around the adds and kills them.
Plague Siphon is what Lich King gains whenever Necrotic Plague jumps.
While it might seem simple that whenever Necrotic Plague jumps, Plague Siphon stacks too, the stack size on Lich King's buff doesn't tell how many stacks Necrotic Plague has. For instance, the Plague might disappear if the adds died too quickly and it didn't have any close units to jump to OR separate smaller stacks of the plague can combine into one larger stack.
So, you need to track Necrotic Plague stacks, and when the stacks go high, it jumps around pretty quickly and might be quite hard to keep track off. Thus, something that could track the stacks from combat log for example could be nice. There (apparently) also might be a bug that if the stack goes from 29 to 31 (skips 30), the achievement might not trigger. Don't know if it's fixed yet.
Maybe you could make a suggestion to the author of RaidAchievement.
I suggested the idea to the author of RaidAchievement, but he says it isn't reported in the combatlog. I just responded and tried to tell him otherwise, but if he still doesn't think so, would anyone be willing to give this project a shot?