First, keep in mind that different characters will attack for different reasons, and will do so based on their own alignment and their aggressiveness.
The easiest way to do it is to use the RAIN->Sentio Characters->Markup Creatures menu. Make sure your custom model is selected, then use that menu to add a “Good” creature aspect to it. From there, the Darkness Warlord and Necromancer will attack.
Each of the Getting Started Guides describe this process, and describe the rules for when the characters will attack.