There would be a real-life time gap between seasons, so if you think a returning audience is going to buy into the tension of a major action setpiece in the season opener, may I direct you to the face-off between Bard and Smaug at the opening of the Battle of Five Armies.
In other words....having seen a glimpse of 'Melkor Triumphant' at the end of the 2nd Season won't be what is destroying the tension. Just another log on the cheerfully burning fire.
The timeline for Episode 13 would be as follows:
We show the Feast of Reconciliation up on Taniquetal, possibly as a 'view from the outside' rather than being close enough to hear conversations. We then go down to the plain below where we see Melkor and Ungoliant attack the Trees, with the result of the light of the Trees beginning to fade away as Ungoliant's Unlight blocks it. As some waxing and waning is quite normal, no one notices/panics at first...maybe someone comments about how 'oh, already Laurelin fades? I must have lost track of time!' Then...the Light keeps fading. Now, we see the awareness of what has happened kick in and people panic.
We see Melkor and Ungoliant flee the scene as the elves/Valar/Maiar up on Taniquetal rush down to find the damaged, diseased, dying, poisoned, sick Trees. Yavanna touches a branch, and it just breaks off. Nienna weeps (she's going to be doing a lot of that....)
We see the panic of the elves in other places as the Light continues to fade. The Teleri on the shore see the dimming of the Light through the Calacirya, and look up at the stars. The elves of Formenos light a bunch of torches and act quite fearful of the darkness, while Finwë keeps his head and looks south, trying to figure out what happened.
Back to Ezellohar, where people are trying to figure out what to do.
ULMO infers Melkor's involvement based on the fact that the Ring of Doom is wrecked - that has his signature all over it, though how he attacked the Trees is *not* clear. How could he have done this? And where is he now?
The Darkness heads north. [Oromë and Tulkas pursue, but are thwarted - beating the air with their hands as they are stuck in Ungoliant's unlight?]
Finwë clearly figures out a threat is coming, and orders everyone present in Formenos to get out. He tells Nerdanel to go to the Valar and fetch his sons - all 3 of them. She asks something like, 'What about you?' and he indicates that he will stay and face the coming storm. [We see...?]
[Return of Tulkas and Oromë with news that Melkor is headed north?] Someone (Varda?) realizes that the Light of the Trees is still contained in the silmarils. Yavanna claims she could rekindle the dying Trees with this. Fëanor panics, realizing that the Valar are discussing the destruction of the silmarils. TULKAS turns to ask Fëanor 'who could deny Yavanna?' and clearly expects him to pull the silmarils out of his pocket right now. Aulë chastises Tulkas for being hasty. Fëanor gives a speech in which he tries to articulate what they are asking of him, and this includes the claim that he will be the first elf slain in Valinor if they demand this of him. Mandos says, 'not the first!' and Fëanor (unnerved) finishes off with the contention that if the Valar FORCE him into this, then he'll know he is truly in a cage.
Nerdanel arrives, with the news that Formenos is under attack and Finwë calls for aid.
Fëanor and his brothers [and Tulkas and Oromë?] rush north, only to find Finwë's body amid the burning wreckage of looted Formenos.
[THIEVES' QUARREL, where it is revealed that Melkor has the silmarils.]
Final ending shots of the last light of the dying (now dead) Trees falling on Fëanor in Formenos, and (possibly) the triumph of Melkor taking the throne in Angband with Gothmog as his right-hand-man is marred by the pain of his burned hand.
So, my two issues are - what is the climax, thieves' quarrel or Fëanor's refusal to give up the silmarils, and....what is Tulkas doing, seriously? How is he at Ezellohar *and* chasing Melkor?
hinthint