With the new Legends of Runeterra expansion bringing in multi-region cards, a lot of people have questions about how this mechanic will work, since we have a two region limit at the time of deckbuilding.
Which regions am I using in my deck?
To know which regions you are using in your deck is very easy: ignore the multi-region cards. Your deck will be defined only by the cards with one single region. If you want to build a Teemo Fizz deck, the first thing to do is to add at least ONE Bilgewater, Bandle City or Piltover card to define your regions, like in the picture bellow:
After the regions are defined, you can add any multi-region card that has at least one of the regions you've chosen. But since both Fizz and Teemo are Bandle City, if you want it, you can also combine any other region with Bandle City, as long as you don't use any Bilgewater or Piltover card to build a deck such as Bandle City Demacia.
That brings some complications to some decks. As the multi-region cards don't count for the deck's regions, it is not possible to put only three copies of those cards if they are not part of the same region as the other 37 cards. In reality, that means loyalty decks such as Teemo Ionia, which use only 3 Teemos to get the effect of Kinkou Wayfinder, will need to include an additional card from Bandle City or Piltover just to include Teemo.
Do multi-region cards work with Loyalty and The Bandle Tree?
Riot hasn't confirmed anything about loyalty effects, but with what we have of info so far, yes. They count as two regions, including the loyalty region, so they can be included in that type of deck. This helps compensate for the fact that loyalty decks will have to include additional cards, as cited above.
Regarding The Bandle Tree we can expect the same thing: summoning a multi-region unit will count as two regions. That way, Bandle City alone will get many units to speed up the condition for this Landmark effect, because to the day this article was written, only Bandle City/Shurima and Bandle City/Shadow Isles multi-regions hadn't appeared yet. Maybe this will make this Landmark quite dangerous, but with a great deckbuilding cost, as you will be using units which don't really help you that much.
With this new mechanic, we will see lots of new decks being formed and using multi-region cards.
We can make new champion combinations using the features we want, for instance, having Teemo in a region with more defensive cards, and the possibility of breaking away from Freljord to have protection in a shrooms deck.
Besides that, we will be able to recreate old archetypes using new cards from other regions, such as the Scouts deck which will now be able to be made using Bilgewater, Demacia or Bandle City, and probably will work well with any of these combinations. This for sure will bring a lot of diversity to the game, making it possible to also use a lot of varied cards such as tech cards, to win in specific metas.