Splinter of Darkness

[kcwong]

To answer the OP:

To get the splinter, you need to kill undead/demon with Moro Dark, while using no undead/demon on your side. Each battle with decrease his dark power by a few percent, and when it reaches zero, talk to Moro Dark.

I found that you need to save the whole nameless island and demon island, plus a bit more undead stacks in earlier islands (e.g. those ghost ships and haunted castles) to unlock the splinter. If you haven't then you may find yourself a few percent short.

The Black Helmet gives a HUGE boost to no. of BKs you can command, so it's very important.

For healing undead, you can use Time back as well.