an easster egg unit is not available by default and not buyable in upgrades, but it is
- in one of the campaign maps hidden somewhere
- it can be get if you eg. mend a specific civilian, or if you convert one
- the clues for it is hidden somewhere (in help, or in other campaign description texting)
- hard to find
- if you have it you can build it as it were an upgrade bought by gems
- practically it can be on the bottom of an upgrade column, but the cost is not gem, but it say "Easter egg, find it".
- when player gets it, it gets "unlocked" in the upgrade columns.
- optionally it can be common/rare/unique:
-- common=many player can find it,
-- rare: maximum eg. 20 player can find it,
-- unique: only one player can get it (no one else)
players not having this upgrade but seeing it in a battle will get insane to get it
