[Maledictor's Doom doesn't need attack rolls; it automatically takes effect. That said, you did the rolls right. When attacking multiple foes, you always roll attacks separately and then damage once which applies to all of them. THAT said, neither of those powers actually do any damage (although Whispers of Defeat will do 9 damage to them if they miss with their attacks). Did you want to push any of them with Whispers of Defeat? I've assumed not but let me know if you do.]
Derath's chants seem to deeply unsettle the beasts (-3 to hit for all of them). Two of them - the one he's challenged, and the one he's attacked - snap at Merik, while the other attempts to pounce on Veonis.
E7 panther:
Rolled 1d20+8 : 10 + 8, total 18
Rolled 1d6+4 : 3 + 4, total 7
E9 panther:
Rolled 1d20+8 : 12 + 8, total 20
Rolled 1d6+4 : 2 + 4, total 6
H5 panther:
Rolled 1d20+7 : 3 + 7, total 10
Rolled 2d6+4 : 5, 3 + 4, total 12