how can I get this?
Hello, I have not made this modification public as of now.
Because a new game version is expected to release to the public soon, I want to wait until I can confirm that the feature will still work in the new update.
Originally it was my plan to develop a character mod to accompany and exhibit this feature, but that quickly became too ambitious for me.
After the new update releases and I have made everything work with the new code I will make the modification public.
Currently the modification includes:
- The titular Summon feature (+ some new attacks)
- Full integration with Blured’s VT+
- AI vs AI gameplay.
- Various bug fixes.
- New modding tools such as the ability to specify a modded unit’s ID & other aspects. (I expect these to either break or be deemed irrelevant in the next update)
I have not had the opportunity to work with anything 0.12a related yet, so I do not know of the new VT+. The current thing I have in place just checks for an [ID] tag in the Class file when loading the mod. If an [ID] tag is not included then an ID is added as normal. 
Could not find a way to ensure that modded units would always have the same ID independent of load order, which they would need for the "Summon" skill to work, so I use this for testing.