This is very simple. When all menhirs are destroyed the silent king counts as a character for the purposes of the Look Out, Sir rule. And because he has more than 9 wounds he can be targeted just like any other enemy unit. No look out sir protection. His wound characteristic doesnt change when all menhirs are destroyed.
Triarchal Menhir
While this unit contains any Triarchal Menhirs models, it does not count as a CHARACTER for the purposes of the Look Out, Sir rule and each time an attack successfully wounds this unit, that attack must be allocated to one of those models. The destruction of Triarchal Menhirs is ignored for the purposes of Morale tests. If Szarekh is ever destroyed, any remaining Triarchal Menhirs in this unit are also destroyed.