I have thought of posting this since last month when I saw a hit of it on my blog. Perhaps the persons who looked up for it hace found the solution, here are some hints to you noobs, mmwah!
The easiest way is obtaining them as free gifts, example here from King's Trial.
At the introduction of heroes, everybody has Jarvis (my dashing booboo huahaha), so when he reaches level 25, he can take your troops to the Hero's Altar in the Dungeon to obtain the soul splinters of Camilla and Lancelot. I have not tried my dashing booboo's strength here. I am just a casual player. Perhaps later, or later, or later ...
The most tedious way to obtain soul splinters, yet the only way to obtain it if no free gifts event is to place yourself in the leaderboard during events. Do not ask me (kuahaha!!!)
And lastly, we can obtain soul splinters from Ancient Sanctuaries level 25. Watch out for events that are not sanctuary event but grants you level 25 sanctuaries and a chance (chance, so do not cry if you do not get any) to obtain soul splinters.