Read My Little Dashie 1-4 First
In everyponies life you realize what you are supposed to do, your destiny. The 2 princesses, Celestia and Luna thought they had already lived through there destiny, it is still far from finished. After the mane 6 died the princesses tried to find the reincarnation of the spirits of the elements of harmony, to no a veil. So when the inevitable 2nd rise of Sombra, they would be able to stop him, without cadence or Shining Armour. After Sombras defeat, and his banishment to the frozen ice of the arctic, of course there was some evil beings that wished to bring him back, to worship him. No other pony, nor any other being wanted him to return more than Forest.
After Brian died no pony ever suspected him as being a human, ever again, as a matter of fact everyone forgot about him. Brian had suspected him as a human, he was much more than brian could of ever imagined, a necromancer. As it turns out Forest actually did have amnesia, but after overhearing rarity saying "i do really wish that the crystal manes where permanent, they looked absolutely divine on all of us", this suddenly made him remember his plans to eventually raise Sombra from the dark and freezing ice. As sudden as this random remembrance occurred, it was replaced in his head with dark, but gleaming purple eyes, he didn't recognize them until the whole figure appeared in his mind, and then said "ha, Forest my most loyal worshiper, why have you stop praising me, your soon to be dark overlord?" Suddenly he remembered this voice, it was deep and booming, in the sudden remembrance of this voice he said, "Sombra, my lord it has been to long!" At the sound of this Sombra just chuckled, uncontrollably. After what felt like a full hour of this eerie laughing, it stopped as suddenly as it came. A very sudden pain came unto his head, it was as if something was stabbing through his snout. As he looked in the mirror he saw a red horn on his head, not knowing how it got there. Lunas beautiful night was just fading, as Celestia was raising the sun, she noticed forests newly acquired horn and stopped dead in her tracks. She instantly summoned her sister Luna, "HOW DARE YOU DO THIS, GOING INTO THE FORBIDDEN ARCHIVES TO TRY TO BRING BACK SOMBRA!" exclaimed Luna, using the royal canterlot voice, booming over every other sound on this day. That is if you would even consider it day, the sun was only a quarter raised, as the sight of Forests horn had made her stop the raising of the sun. He had heard lunas shout, but it just went through his head, as he was deep in thought he was surrounded. All he could think of was run, so that's exactly what he did.