This story is a sequel to Breakfast Rewards
Private Iceland is going back to work after a long stay in the Crystal Empire hospital. He's more than eager to, wanting to forget his dreadful experience that landed him there.
However, it turns out that his enemy had put a hidden spell on him. The spell backfires and brings Private right to King Sombra's hiding place.
King Sombra is just as surprised as Private, but not as startled.
This story is part of The Equestria Tales series.
Side-story in "Return of The Dark Ponies" story arc.
Story edited by Hail King Sombra.
Cover image by Hail King Sombra.
Chapters (1)
Hrm, this one was a little confusing to tie-in with the other story, where he's escaped and living life. His positions on a few things feel completely reverse, which makes his motives feel skewed, in a way? In this story, he speaks of how he hates the Two Sisters, but in the other one he's obviously got a thing for Celestia, though it seems purely physical. In the other story, he also comes across as just a bit more ambitious - something he doesn't show here, though showing him enjoying life is interesting and neat. So, I presume Sombra has his own side story/novella maybe giving even more expansion on how he spends his time?
For a story with the random tag, this one had a very consistent and amusing atmosphere that even felt pleasantly spooky a few times, like towards the ending. I feel like Sombra has a weird way of caring about Private Icy here, which gives some context to the other short pieces with them. It says in the description that they are enemies, but so far they haven't felt too much like it, and it's kinda cute.
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I would not take the line "my hatred of the two sisters" too seriously, it was meant to be him mocking his enemies, making fun of them if you will and poking fun at how many would think he was just brooding somewhere in a corner, while he's actually just enjoying himself far away from them while making his plans ready. Also this story takes place a little earlier than A watchful shadow, we could say at this point he's still getting a feel for having a body and is still kind of in a sensory overload, but by the time of A watchful Shadow, he's starting to get back into the groove of things.
And yes Sombra has his own side of the story, some of it is in fact expanded on a bit in Last of the Dark Ponies, and I have considered writing more stories that happen between Last of the Dark Ponies and Possession.
I am bad at tags.
If you have better suggestion, I'm all for hearing it. I swear the tags are my mortal enemy on this site. 
And yes, Private and Sombra have a complicated relationship, Private sees him as an enemy, King Sombra sees him as a friend/servant.
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This and the writing guide are a pretty good resource for tags.