Helya is one of the banished creatures and is stuck in Hellheim forever opposing Odyn’s every word. They are long and bitter rivals who seek out to destroy each other but cannot since they are bound to other dimensions. Odyn has had a long history with Helya and he seeks the help of you the adventurer to destroy her.
This is why you need a trial of valor boost in order to defeat Helya and banish her once and for all. You are not only going to face Helya but Odyn himself, if you can prove to be a worthy competitor and defeat him on the throne in Skyhold then you are worthy and strong enough to go and face Helya herself.
The trial of valor boost is one of the shorter raids but it does not make it any less important whatsoever than a full raid. The lore that is happening inside the raid sums up one of the main Legion story lines and it is a raid that everyone has to experience since they quested through Stormheim.