This.You don't take any Ld tests!
Its your enemy that has to. And if they fail, be reduced to 1WS for the duration of that Assault Phase.
Just one not five. An enemy assaulted by one or more UNITS equipped with Death Masks must pass a Ld test... blah blah blah...
There is no negate modifier to the Leadership test, it works exactly as stated in the rules.