Spoiler: Dorni
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This is Helios's first contact with a seemingly natural font of magical energy, so a lot of what he's doing is guesswork. Based on the "size" and potency of the font, sealing it is almost certainly beyond him. As to diffusing it, Life magic seems to cancel Death magic, so releasing Life Magic into the air would at least delay the effects. Doing this would be finicky at first, but would probably get easier with use; if he works on it, Helios might figure out a way to simply release Life Magic into the air in controlled bursts by his presence alone. This would tire him, but it wouldn't be too much of a hassle.
Of course, the two concerns at the moment if he did this are Manchineel (who may be afraid of this place, but still counts it as part of her domain) and the Suneaters (who might wonder where Helios is going and try to track him). Still, those are tangential to the problem at hand.
OOC: Diffusing the Font would be a short-form Downtime for Helios, which would eventually dovetail and give bonuses to his Mystical Research Downtime. However, until/unless Manchineel or the Suneaters are dealt with, investing in this Downtime would almost guarantee a Crisis.
Extra: The two are roughly similar... in fact, it would take a little work, but the two forces would probably cancel one another out.