It's a game setup by the dev team not through the regular system. With staff and lots of organization involved.

Basically but it's through the GvG system.

Eternal Battles are a group of volunteers who facilitate GvG matches that are then streamed with caster so people can watch. However GvG has been going on for months and it doesn't need Eternal Battles to set up.

You just need to speak to, offer a challange, call out or ask another guild if they want a friendly match. Then set it up with the devs. The map is your choice and it'll be open for however many days you ask. You just coordinate with the enemy team when you are all going to turn up and then queue.

If WCA (or any of your sub guilds) are on speaking terms with another guild you could set this up easy. I mean it takes a bit of coordination but set a time, a date and a map and it's game on.

Lots of guilds even use it for training by pitching their sub guilds together and testing out tactics or exploring new maps.

It's basic, it really needs improving but it works and guilds are using it and have been since it was introduced.