It's not only tallars event, it's "Magical Storm" and it takes place in many different battlefields and even on GBs on plateu.
It's always one BF at a time and the magical strom brings usually new rules or some buff,
on arena for example it disables use of mounts, in CC it makes teams 6 vs 6 with only 300 points to score and enemy killing give points and handing in batch of crystals will give you buff.
I really believe this event is a way to make BFs more attractive and wish we could start this event sooner than next year