Adding Serpentines

[Blademun]

Alright, I need some help. I wanted to add Serpentines to the Byzantium unit lineup. After editing in their ownership of them in the export_descr_unit.txt, I checked for them in custom battle but they were not there, not even a extra card.

is there something besides ownership I'll need to edit to give the Byzantine's the serpentine and have it show up in custom battle? ( I don't care what textures they use. THe padded armour you see them wearing is fine)

[Aegis]

That should be enough to allow you to use Serpentines in custom battles, though you'll need to make them recruitable from buildings to use them in the campaign.

However, it's the bare minimum, and without unit cards, it'll use a default card (looks like a peasant dressed in black with a knife).
To get proper unit cards, you need to do a couple of things. The first is to copy the unit card file (.tga) from a faction in data\ui\unit_info\ to the faction you want to give it to (in this case to data\ui\unit_info\byzantium). For example, take the card from Poland (data\ui\unit_info\poland\ee_serpentine_info.tga) and copy it to the Byzantium folder. Next, you need to copy data\ui\units\poland\#ee_serpentine.tga to data\ui\units\byzantium. Keep in mind though, when you change ownership, use the serpentine that matches whatever faction you took the card from (if you use Poland, that'd be the EE Serpentine unit).

The next problem you'll encounter will be that the crew lacks proper textures (I'm about 95% sure they do anyway) and you'll end up with units that look sort of dark metallic due to the shared gloss map and lack of texture to cover it up. Again you can use already existing textures from other factions, but for that you need to modify the battle_models.modeldb file. If you open it in notepad, you'll quickly discover that it's not the most user-friendly file to edit manually.
There is a thread or two about how to edit this file that you can try reading. Make sure you create a backup before you try anything though, as mistakes are easy to make and will crash the game until fixed. I can write a line for you to paste in, but I don't have time today.

[Blademun]

Well, it seems somehow, my game has been broken. I only made that edit to the unit decrip file, and I opened the modeldb file with notepad but didn't do anything to it. Now my game crashes with a error with some hex numbers and a statement saying the memory could not be "written" or something along those lines. It crashes the moment before I begin battle on the tactical map.

Since I didn't make any edits to the modeldb file, I didn't bother making a backup when I peeked at it. :/ No clue what could be wrong.

[Aegis]

I've had that too, and it was not related to the modeldb file. Just reverse any changes you've made to other files.