Scripting a new summoning spell

Let's start with setting up the creature to be summoned.

Setting up your summoned creature

Start the construction set.
Select the master files you wish to use.
Don't set an active file.

Click the Creatures tab.

Right click on the alit. Select Edit.

We'll stick to Bethesda's naming convention and change the creature ID from "alit" to "alit_summon".

Click the AI button. You can copy the AI settings from another summon, such as "ancestor_ghost_summon"-- change the Fight setting to 50, and the Flee setting to 0.

Select Follow in the dropdown menu labelled Add Package.

In the AI Follow Package window, enter player in the Target text box.
Leave Duration at 0.
Leave Follow to unchecked.
Click Save.

Time to remove the old AI Wander package.
Click on Wander in the AI Data window.
Click the Delete key on your keyboard.
The AI settings for "alit_summon" should appear as follows:

Save your new creature. You'll get a warning about the changed ID-- choose Yes.