Hell Flare

Hell Flare (黒炎獄) is a Composite Skill consisting of Ranged Barrier, Fire Manipulation, and Black Flame, created by Benimaru.

Description
It's described by Rimuru as being akin to Ifrit's Flare Circle, except with an even larger range. First, space is secured with Ranged Barrier, then the molecular motion inside is accelerated with Fire Manipulation to produce high temperatures. Finally, the excited particles inside the space acts as fuel, and the Black Flame converts it into a searing flame that burns whatever is inside.

To perform the attack, Benimaru produces a black spherical flame in his right palm. Increasing the diameter of this ball to a meter, he shoots it at the target. The ever-accelerating round is able to chase any fleeing enemies at a speed of around 600 kph. Those struck are immediately engulfed and burned in a 2,000 ~ 5,000-degree fireball, without leaving ashes. However, the terror of the flame didn't end there. The ball also absorbs the magical energy of the enemies struck and uses it to fuel its growth. Rather than a ball, it turns into a dome with a radius of 100 meters.

Unlike a nuclear explosion, Hell Flare doesn't cause any damage outside of its radius. Even if the barrier is released, there is no shock wave that goes outside. Range specification can be performed, it seems the purpose for increasing the inside heat synergistically is to make the heat inside unbearable. So once confined inside the boundary, there is no hope for survival.

The number and size of the fireballs created depends on the magical ability and the amount of magicules in the user's disposal. Benimaru, for example, was seen creating six big ones in the anime against the Orc Disaster's subordinates, killing thousands of orcs in the process. However, the Orc Disaster himself was able to survive a direct hit with one due to his fire resistance. This is because the skill is designed with efficiency in mind. It's meant to do its job within only a couple of seconds, and the fire can't be maintained for extended periods of time like with Ifrit. If the target has enough resistance, they can easily survive the blast.

Fortunately, Benimaru avoids this setback later on after evolving into a Fair Oni. With the help of the Generalissimo skill he acquired as a gift from Rimuru's Harvest Festival, he's able to sustain Hell Flare even longer, and is able to increase the heat of the flame to overcome any resistance or regenerative abilities, easily exterminating the likes of Charybdis.

Rimuru, owning all the base skills, is able to use it as well. With the support of Great Sage, it's possible to reduce the size of the dome and produce heat going well over hundreds of millions of degrees celsius.

Trivia

 * In That Time I Got Reincarnated as a Slime: Isekai Memories, the "Benimaru [Samurai General of Hellfire]" unit uses this as a Secret Skill.
 * In that same game, the "Rimuru Tempest [Swordsmanship Forge]" unit uses a similar Secret Skill called Binding Hell Flare (縛糸・黒炎獄), which is a combination of Arcane Thread Fetters before following up with Hell Flare, referencing how Rimuru defeated Ramiris' Elemental Colossus.