The Titans are formed from fire, nature (earth) and ice (a form of water). That's three of the four classical elements of ancient myth. The fourth element is air, which is represented by the Amulet of Daylight being upgraded with alien tech to make it a Magitek device that can match the Titans.
Another ancient myth speaks of five elements: earth, water, fire, air and aether, the material above the sky. Both air and aether represent the Akiridion extraterrestrial element to match the other three.
There's also the fact that Jim's last name is "Lake" and Nimue, the Lady of the Lake, is the crafter of Excalibur, which Jim now wields.
Nari kills Skrael in a Mutual Kill to prevent the three Titans from uniting. If anything, even she should've known this as well. It's later revealed that if only one Titan succeeds in uniting with the Heartstone, the universe is done for, this making Nari's death look like a Senseless Sacrifice. Then why would Nari tell Jim to find the Krohnisfere and use it to travel back in time to make things right? Nari planned ahead all along. Nari has known just how dangerous it is if one Titan succeeds, but if she were to not make it once she faces Skrael – her opposite counterpart – then the universe is to have a better chance at survival by showing the heroes just how irredeemable and hypocritical Bellroc and Skrael truly were (as in revealing their true intentions with the Titans), hence why she defected the Order in the first place, and why the Genesis Seals cannot EVER be broken. Nari needed Jim to see just how life-threatening the Titans are since they've barely managed to stop Bellroc without her, and that he needs all the heroes working together to protect the universe and claim Excalibur in the right hands.
Jim and Bellroc's animosity throughout the plot, and It Makes Sense in Context that Bellroc has become more of an Arch-Enemy to Jim in the movie than Gunmar ever was in the series:
Gunmar was Jim's enemy mostly because the Trollhunters were a bane to Gunmar's entire reign, so Gunmar was mostly Jim's enemy professionally. Bellroc, however, has left a lot of impact on Jim after the events of Wizards (from critically injuring him, brainwashing him into attacking his friends, etc.) In general, what Bellroc has done to Jim was far more personal than what Gunmar could've accomplished, thus Jim's hatred and defiance towards Bellroc in their interactions.
According to Aja, Akiridions usually give birth to three to five children. Since Aja and Krel are the only children in their royal family, does this mean they have a deceased sibling?
