If you want to prevent bugs, create a duplicate folder of your whole Magic directory, then rename all GY files as the Custom files are named (MagicCustom.exe, ManalinkC.csv, cardC.dat) and run My2000. Then, since it is buggish, keep a backup copy of your origibal Magic.exe. Then create a new Folder in your desktop with the following files:
csv2dat
magic.exe
Shandalar.exe
SkyMagicEditor
manalink filters
ManalinkEh.dll
ManalinkEx.dll
ManalinkEh
BinaryCards
manalink
BinaryCards.dat
move in all the edited files with My2000 and rename MagicCustom.exe, ManalinkC, cardsC as Magic.exe, Manalink, cards.dat. Open both this Skymagic Editor and the one inside your main Magic directory: open the same card and manually copy the exe part of each card from the old Magic file to the new Magic.exe that will become MagicGY.exe
Remember to save often both the card just copied and the whole changes or they won't take effect. When you open Skymagic editor, it may give you an error message about Magic.csv. If it does, just open that file with Excell and save it. That should fix the trouble.
I know that it looks very complicated but My2000 will save a lot of your time to copy card art and CSV part, but is occasionally buggish for the exe part and doesn't allow to edit some hard-coded cards. So, using Skymagic editor is much more secure and time-saving rather than using any other program. Hope it may help you!
PS: there is another error in your
Teferi's Veil : even if it can't be used in combo with
Ball Lightning, it may still be used for other purposes (as
Wrath of God & stuff...) but phasing creatures will come back each time under human player's control and not under their owner. I'm sure there is just a flag to fix.
Also, any hope to have Doran working in Custom?

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