Eydis Darkbane and Fjola Lightbane: Legends or Myths?

The Grand Tournament cards Eydis Darkbane and Fjola Lightbane invoke decks strong with buffing mechanics. Whether these cards warrant entire decks or represent very strong candidates for the 3-mana position remain questions. The questions that remain for the casual gamer are whether these legendaries are worth the cost and for which classes and decks these cards make the most sense.

Cost:
As for cost, on average, each pack of cards nets ~94 dust, so purchasing $20.00 of The Grand Tournament (TGT) packs should allow crafting of 1 of these legendaries assuming that you dust most of the cards opened. As such, this isn't a huge investment. Further stats on opening of packs can be found here and my own work here.

Strength:
Both cards are notable for having good attack and health for 3-mana. The archetype of a 3-mana card is arguably the Spider Tank with 3 attack and 4 health. Eydis and Fjola have those specs plus the added benefit of "3 damage to a random enemy" and "gain divine shield" when "you" target either with a spell. These are both good effects. 3-damage aimed randomly is rarely useless and usually helpful, but it will cost you some mana and usually another card to do so. Otherwise you will be required to target Eydis, perhaps creating some opportunity cost compared to another minion you'd rather target. As for Fjola's divine shield, this lends itself better to a deck archetype familiar to Magic the Gathering players called "Infect," where the goal is to relentlessly upgrade and attack with one or two specialized minions. The option for a "Hexproof" style of play is also available from the Divine Shield itself. Using either of these strategies would require a deck with at least some buff cards, such as Spare Parts. Prototype decks can be found that include both cards, but my sense is that this card will become a Priest standard and a one-of (probably Eydis) in aggressive decks with some buffing spells.

Staying power:
If you can get either of these cards out early in the game and target them with a buff they have some chance of remaining in play and may offer reasonable board control from turn 2 (with the Coin) to 5. Cards costed 3 or less with Charge have 3 or less attack, except for Arcane Golem- a card that I have not seen a lot of on the ranked ladder. The real concerns are Polymorph effects and Silence effects which still are threats available for 3-mana or less. After and during turn 5, things start to get more difficult for Eydis and Fjola, but they will certainly require some resources before quitting the board.

Summary: 
Eydis Darkbane and Fjola Lightbane have created a great deal of excitement. My sense is that for a casual player these may be good investments, especially if playing buff-heavy classes such as Druid, Paladin or Priest or a Spare Parts-heavy strategy.