( If the caster dies, the dancer receives 100% damage and their mana is reduced by 100%. This effect cannot be dispelled.)
Yes, it is a negative effect to keep her from being too OP. Similar to the Easter heroes.
Is Sechat mana affected too?
Yes. G. Gazelle dying while Dance of Spirits is active is one of the exceptions that can bypass Seshat’s resistance (also Red Hood’s minions) to otherwise all direct mana reductions.
The other exceptions are listed in these release notes: