Here's another event that offers hero soul splinters, the Elite Recruitment. This is when the recruitment of star level troops is allowed in the main and second cities, with reduced recruit time.
bla bla bla (pls find time to read because I am just highlighting on the soul splinters part hehe...) Use at least 10 level 3 speed ups to randomly get either soul splinters or equipment fragment. Random!
Do not be doubtful, just go with it.
That's all folks! And do not be discouraged if you did not get a soul splinter. Try again.