Okey...Les start with a couple of things.
First of all. Illaoi can rip the soul of people without actually testing them, or at least something similar. I've recently re-read Shadow and Fortune and this is what i've found.
"She shielded her eyes as the light billowed outwards like writhing tentacles and where it met the Black Mist it stripped it bare, exposing the twisted souls within. The sinuous light pulled the dead onwards, purging the baleful magic that cursed them to undeath so very long ago."
"
Hecarim's blade drew back for another strike, but before the blow could land, the light finally pierced his veil of darkness. He bellowed in pain and fell back from its burning touch. The dark rider's outline shimmered, like two picture box images wavering in candlelight on the same backcloth.
Miss Fortune caught a fleeting glimpse of a tall rider, armored in silver and gold. A young man, handsome and proud with dark eyes and a future of glory ahead of him. "
This is for the part 4 of Shadow and Fortune. Basically the climax when Nagakabouros power purge the Black Mist
About the tentacles. Those are not "water" tentacles and his God is certanly not a terrestrain god. Much less a water god. The description of the tentacles is always like this:
"Miss Fortune turned and saw the spiraling statue bathed in blinding radiance. White light smoked from its surfaces, and shadows fled from its touch. She shielded her eyes as the light billowed outwards like writhing tentacles and where it met the Black Mist it stripped it bare"-Shadow and Fortune
"She reached out with her mind and called forth the energy of the Mother Serpent as she swung the giant idol forward. A glowing mist vomited from the icon's mouth and swirls of blue-green energy formed around the Mother Serpent's face, solidifying into ghostly tentacles. Touched by gold, these tendrils were as beautiful as the sunrise over water, and as horrifying as the darkest undersea abomination. More tentacles grew from the icon, replicating around the room as if born from some unknowable mathematics. Exponentially they grew larger, and somehow each one's growth seemed to hold all the promise and horror of the world."-The Burden/Illaoi's backstory
I think it's pretty clear that they are mystical. Not water tentacles.
Also. Lets start talking about the Test itself.
The Test of Spirit that Illaoi uses it's a way to put a person "in motion" the concept of "motion" in Illaoi's religion is a bit complex. But i'll try to explain it. To the people of Buhru everyone should follow what their hearts desire and accomplish their true destiny. It doesn't necessary has to go against the wishes of a nation, or a code, or is related to good or evil. This can be seen in some of her quotes. For example. her taunt against demacians:
"You are slaves enchained by lies when we were born to chase our desires!"
"What is truly noble and good would never ask you to serve another!"
"Honour? Duty? Ha! What your heart demands is all that matters."
"No love is impossible, my god demands you follow your desires."
Basically. The concept is that people should follow what their heart desires and only serve that purpose. That's why people like Braum or Jinx are ok for Nagakabouros. They are doing what they want in life (Braum helps people, and Jinx blows up people) and what they serve also serves their purpose. They are also emotionally healthy (Well...At least Braum is) yet people like Garen or Katarina, even when they are partially doing what they want (serving their nations) Their devotion is also going against their wishes. Because both love each other but can be together because of their loyalties. This is also rule with concepts like good, evil, ect. Illaoi doesn't necessary want you to be evil nor good. She wants you to do what you want to do with your life. Akainu obviously has the willpower, but that's simply not enough.