So the idea for this originally came when I was trying to come up with support cards for my Archetype of "Discord" monsters, while making the field spell I realized they didn't have a proper boss monster and decided to fuse the two into "Grand Discordian King Xaos" a monster that must first be played as a field spell, kind of a reverse of how ED/Pendulums typically work
Of course he wasn't originally given the "Domain" Sub-Type and the wording was a bit messier, a while later I expanded on the idea, figured out a more reasonable way to word it, and began making a set of "Domain" monsters for each attribute along with their own version of terraforming.
Domain monsters (so far at least) tend to be high level with "Domain effects", as I've come to call the effects active while in the field zone, that support and help them set up for when they become monsters, while they are special summon-able by other means the advantage a few of them gain from being field spells first can make it worth playing them as expected, especially with their own search card. The all have the same layout to their effects;
"This card cannot be Normal Summon/Set, this card can only be played (from your hand) as a Field Spell card with the following effect;
- (insert Domain effect)
If this card is destroyed while in the Field Spell Zone, Special Summon it. (insert monster effect)"