(I’m not going to bother with things just about ‘oh, she and Shining are married/this can’t happen/she and Sombra hate each other/blah blah blah’ I don’t need to deal with those types of comments, there is such a thing as a change that takes place over time and a change in opinion of someone, people, just because Cadance loves Shining now doesn’t mean she will always, get over it)
 
 
Headcanon : After Shining Armor betrays Cadance(or as Cadance feels he does), she breaks off their marriage and after Shining Armor leaves the Crystal Empire, Cadance falls to depression and anger, anger at Shining leaving her, anger at Chrysalis for taking him from her, and anger at herself for not being good enough to compare to the changling queen in the mind of her husband(note : this is just how Cadance sees it, not positively how things actually are), her darkening heart causing the remains of King Sombra to shift and start, awakening the king, who began to slowly work his way into the princess’ mind and taint her heart with the promises of love she wanted/needed, King Sombra took Cadance as his queen when she actually delighted in all the dark ideas he had, the princess’ love’s betrayal had already darkened her beyond repair, and she seems to be enjoying herself as it is, waging war when she sees fit and breeding a dark kingdom that she seems to desire so
 
Notes
 
Because she is a pony centered around love, Shining leaving her darkened and corrupted Cadance’s world view far more than it would any other pony  
She really just wants to be loved, Sombra gives her what she wants/needs  
Cadance grows crystals out of her over time, simply because I liked it  
points Sombra and Cadance made a foal~  
Shining Armor and Cadance’s marriage was always destined to end in heartbreak for the pink alicorn, but Celestia and Luna allowed the marriage because they wanted to teach Cadance some sense of ‘you can’t always have what you want’
 

 
The little pony-Me  
Princess Cadance and King Sombra-My little pony