Unfortunately, I see the only way to stop this threat to Felwood is a frontal attack. There are no other ways into Shadow Hold, and I have no means to teleport you inside. The straight path down to the bottom is the only way... and I also know who you should seek out. His name is Fel'dan... Shadow Lord Fel'dan. He is the mortal in power down below... he is the head of the serpent. Kill him and his two succubus servants and return to me. Fel'dan is the one making pacts with the Legion. He must be stopped.
Without Fel'dan, the Shadow Council will hopefully lose focus and start infighting. They will struggle with one another for control and hoard assets for future plans. That will be an opportune time to devastate them and their demon slaves.