(Rimuru POV)
"May the wrath of the gods pierce you. Megiddo!" I suppose my heart really was that of a monster, because not for a single moment did I shy away from committing a massacre for the sake of my friends.
Megiddo was magic that relied on summoning as many low level water spirits as I could control in the upper atmosphere and then use the droplets to concentrate sunlight into deadly beams. It was easy to stop if you knew about it, but I couldn't think of a more efficient way to massacre thousands of people at once.
So, I covered the invading Farmus army with an anti-magic area. I even applied a bit of a Domination effect to give it the ability to purify magicules similar to the way a Holy Field did it.
Then, I began. A hundred deaths. A thousand. Two. By the time the death toll crossed five thousand, the Wise One gave me a notification.
'Report. All hearts broken. All souls of invading army can be harvested. Proceed?'
And with that, they all took their last breaths, collapsing on the spot, dead.
That was fast.
'Report. 20,000 souls harvested. Beginning planned Demon Lordification process. Utilizing Domination effect to hold Harvest Festival off for exactly thirty seconds. Once Festival begins, master will immediately enter sleep mode.'
So basically, I have thirty seconds, then I'm going to be forcibly put to sleep. Just as expected.
Dropping to the ground, I called Ranga out and got onto his back in slime mode. Then, I carried out a demon summoning, using the 20,000 bodies as the sacrifice. Three demons showed up.
"I'm about to go to sleep. Then, most of Tempest will. While I'm sleeping, I need you guys to question Farmus's king and the Cardinal in charge of the Church's Farmus branch. Don't kill them." I yawned. Ten seconds left. Then, the Wise One would name Charybdis, and I'd go to sleep. "Once all that's done, send them back home. Help Risa deal with them."
"Understood. Once that is finished, I would beg of you to employ me in your service, even if as the lowliest of-" I cut the demon off. Sleep mode was beginning to overtake me.
"Talk about it later. Be useful to me now."
"Consider this trivial task accomplished." The demon replied, as Ranga sped off as carefully as possible with me on his back.
…
After sending all of our guests back to Blummund, we'd decided that the best option for us to optimize the chances of Shion and the others being brought back was if I underwent my Harvest Festival ceremony in a special seat that we'd set up in the plaza. Now, Ranga brought me to that seat, and Shuna placed my inert slime body on it.
As soon as they did, my body began changing randomly. Not just shape, but color, too. My unconscious form cycled through all of the colors of the rainbow before settling back on my normal blue teardrop. This was all the tiny stuff that Risa had basically skipped, acclimating my body to its new evolution.
By the way, Charybdis's new name was Chia.
'Report. Harvest Festival begun. Super evolution from slime to Demon Slime successful. Consuming Charybdis, now named 'Chia's' power. Successful. Evolving from type Demon Slime to type Divine Demonic Slimelord. Successful. All bodily attributes have been greatly enhanced. Spiritual Core successfully created and fused into current physical core. Material and spiritual bodies now freely transformable.
Gaining new intrinsic skills. Abridging explanation, regeneration, detection, intimidation, and control-type skills all evolved. Resistances reacquired. Resist spiritual attack and resist holy attack acquired, as well as all prior resistances.
Requesting evolution of Unique Skill: Wise One. Successful. Unique skill: Wise One has evolved into Ultimate Skill: Raphael, Lord of Wisdom.' Everything happening so far was exactly according to plan. But as it evolved into Raphael, the nameless Unique Skill noticed something that gave it pause.
'Report. In analyzing new Ultimate Skill: Raphael, Lord of Wisdom, utilizing similar Ultimate Skill now held by individual Risa, Thoth, Lord of Curiosity as baseline, a potentially dangerous irregularity was detected. Isolating irregularity… Failed.'
Unbeknownst to my newly evolved skill, the framework for the skill: Raphael, Lord of Wisdom, had been created by the eldest True Dragon, Milim's father, Veldanava. As such, that particular skill, like all angel-series Ultimates, had a 'back door' built into it. Anyone wielding the skill that Veldanava created to access that back door could theoretically utilize it to control my soul. Normally, there were safeguards in place to prevent the skill itself from noticing that little tidbit, but the Wise One had gained quite a bit from analyzing Risa's skills, particularly Izzy. While it wasn't quite far enough along to become a Manas, it was at least a bit self-aware.
Now, to protect its master, newly evolved Raphael decided that this back door was dangerous. It attempted to isolate it, contain it, but having been created by Veldanava himself, it wasn't quite up to the task yet.
So immediately, Raphael began searching for ways around that problem. 'Requesting permission from individual Risa to utilize Ultimate Skill: Horus, Lord of Dominion in order to assist in containing irregularity. Permission granted. Interference detected on part of Individual Veldora. Utilizing Horus, Lord of Dominion, and Faust, Lord of Investigation on Raphael, Lord of Wisdom in order to execute forced Ability Adjust on both individuals, Rimuru and Risa. Successful. Soul Corridors between individuals: Rimuru, Risa, and Veldora evolved into Soul Link. Irregularity successfully isolated and transferred to individual Risa under the effects of Horus, Lord of Dominion. Raphael, Lord of Wisdom successfully reconfigured into Isis, Lord of Acumen. Individual Risa's Databank-type Subskills have been merged with Complete Memory, Gift, and the effects of Domination and Unlimited Imprisonment into Mnemosyne, Lord of Archives.'
Finally finished with its own evolution (and apparently both Veldora's and Risa's, somehow?), Isis turned its attention over to the rest of my evolution, just kind of ignoring the fact that it'd managed to master Unlimited Imprisonment to the point where it could integrate it into Risa's new Ultimate Skill.
Gluttony pulled parts from Risa's Horus, Lord of Dominion to become Belzebuth, Lord of Gluttony. Using Unlimited Imprisonment as a base, Isis compiled basically all of the rest of my skills into my third Ultimate, Uriel, Lord of Vows.
Unfortunately, Isis located the same flaw in Uriel that existed in Raphael, so it used the same process as before to consume the flaw with Belzebuth, reconfiguring Uriel into Aether, Lord of Bonds.