The Oversight.

Helios was supposed to get to the divine plane regardless of the outcome of the clash between Aeternus and the GodSlayer. He was supposed to have left for the divine plane as soon as the GodSlayer was fired but he couldn't. Legion planned for a lot of scenarios, but they didn't plan for Stelios to enter the GodSlayer and become ammunition too.

Stelios was supposed to be weakened in his divine kingdom. He was supposed to be so weak that he is unable to answer prayers but somehow he found himself in the GodSlayer. He shouldn't be in the GodSlayer because each strike that Stelios endured also affected Helios. 

Helios is the reason why Stelios' consciousness didn't disperse into nothingness. Stelios is not strong enough to withstand the power of Aeternus himself. Helios had to share the burden of those three attacks because he doesn't want the Authority of the sun to disappear.