Here's one event that offers you soul splinters.
When 20K soldiers have finished being recruited, an ancient sanctuary will appear near your main city. These sanctuaries offer chances to obtain soul splinters. If you did not get any, do not get upset because you could still win other things like maggots, exp potions, or some other things as stated in the event's terms and conditions. One could get up to 15 sanctuaries refreshed. I do not remember how many sanctuaries I got, but two or three of them were raided by somebody else. So how to get 20K recruitment? Just recruit low level soldiers and use your speed ups.
Two corinna splinters.
And one Wiseman just in the nick of time.