At first, Umarak absorbing Ekimu and Agil's light powers out of nowhere feels like an Ass Pull (and still kind of is, in all honesty) until you look at his official description on the website; He was MADE to absorb the power of the Elemental Creatures. Of COURSE he has the power to absorb an attack that was specifically powered by Agil- it's his sole purpose!
Kulta's name. While it SOUNDS cool, it doesn't make much sense (being the Finnish word for "Gold", which is a word associated with positive characters and isn't even in his color scheme), so I assumed it just operated on Rule of Cool. However, when you think about it, it reflects Kulta's desires; he seeks to take and destroy the Golden Masks of Power. Additionally, now that we know he's the leader of a band of pirates named the Skull Raiders, his name suddenly brings to mind a pirate stealing and burying GOLD. In all likelihood, given the kind of culture the Skull Raiders have, he was named after one of the key objects his tribe constantly sought after- It'd be like naming the child of a Buddhist Monk after a word that meant "Enlightenment." Plus, it likely invokes Awesome Mc Coolname in-universe.
While the ending of Journey to One is left on a seemingly triumphant note, Makuta is still alive in the Shadow Realm and still able to keep communicating with his army. Now that the Toa are gone and the only people left to protect them are Ekimu and the Elemental Creatures, Makuta can still have the remaining Elemental Beasts and Shadow Traps take them out and eventually the Okotans as well- case in point, a POWERED UP Ekimu is nearly killed in a head-on confrontation with a Storm Beast, and a single Shadow Trap is able to disable an Elemental Creature. And since Kulta and the Skull Raiders are likely going to dig their way out (especially if Makuta ends up needing their help) and Kulta can presumably create more Skull Creatures using the remains of others, the Toa have only really temporarily stopped the problem.
Umarak's purpose is to drain the Elemental Creatures of their power, an act that presumably kills them. When Agil gives Ekimu his power boost, he's nowhere to be seen afterwards. Later, Umarak gloats that he ended up ABSORBING Ekimu's power burst, and is later absorbed by Makuta. With that in mind, it seems like Agil ended up getting absorbed by Umarak and later Makuta, essentially killing the Elemental Creature of Light.
Kulta is able to create Skull Creatures from the remains of other living beings, such as deceased Skull Raiders and Scorpios. Considering how Skull Spiders can control and effectively kidnap Okotans, who's to say Kulta didn't kill and transform several captured villagers into Skull Creatures?
In Journey to One, we find out that When the Mask of Ultimate Power was knocked off of Makuta's face, it created a portal that dragged the ENTIRE Capital City and Makuta into the Shadow Realm. And this happened when hundreds of Okotans had gathered right around Makuta. Later, when Gali visits the Shadow Realm, we see what appears to be several shadowy Okotans amongst Makuta's army, likely planning to escape through the portal. In all likelihood, all of those Okotans had been trapped in the Shadow Realm with Makuta and were corrupted as a result, and are now all trapped in the Shadow Realm with a megalomaniac and other monsters for all eternity.