I always wondered how the bell worked in mega-games. Strictly by the book, of course, it effects everything. You can also see multiple screaming bells in games like that ringing in the same turn. I always thought they should have a zone of control--a large one, like 60" or something, but not indefinate (Imagine one on a 20 footer table at Games Day).
If the bell is destroyed it is pretty catastrophic--the seer no longer takes the str5 hit as mentioned but both he and his unit take multiple hits in addition to that. The bell also causes impact hits as per chariots when the unit charges.
The grey seer can cast skitterleap, warp lightning, vermintide, pestilent breath, death frenzy, and plague. Death frenzy is a particularly fun spell because your opponent will probably use it to draw out your dispel dice and you, probably wisely, will ignore it in favor of warp lighting and plague--and before you know it he's charging you with a disposable unit of 2-point slaves with 3 attacks each. He can also use it on his weapon teams to make them immune to psychology without liability (since they can never charge anyway).
The grey seer doesn't get a ward save--he gets a 5+ save from the carriage and the option to hide out of sight on it if challenged. If something str7+ gets through the bell's ward save it will destroy it like any other chariot and cause havok on the unit. If the grey seer is killed, the bell and bell-ringer can continue on.
It's definately spooky but in my experience a simple warlord with a rusted halberd is a better choice. The skaven will have a bubble of LD10 coming at you as well as enough points saved to create an extra battle line of slaves with hard units mixed in.
