Tevis was one of the most powerful Hunters and Guardians in general (I mean you took up his bow and it was still powerful enough to pop a super every five seconds) the reason he died was because when he entered the Black Garden, his Ghost got seperated from him and was left behind on Venus. He was still able to form his bow without his Ghost, but that's about it, no revives, no alternate guns or ammo, and possibly no other abilities. He was probably in the Garden for at least a twelve hours on account of Cayde reporting him as missing, rather then compromised, so he could only hold out for so long in a place shrouded by Darkness, and there's also the high possibility that he was drunk, based on his character description. But for the Warlock quest, they were struck by lightning and filled with an ungodly amount of Arc energy and for Titans, they were in the Sunbreaker's forge which was filled to the brim with Solar energy.

It's just an influx of light. Like lots of it. I think it's a way of explaining a surge in power, not necessarily light but charging your light in extreme fashions. The stormcaller mission is you in the middle if a thunderstorm. This is the source of your arc power, so like you're moving around in a field of energy that's surcharging you. The sunbreaker mission is you in the home if the sunbreakers (their forge). Naturally should be an area of extreme light, in order to sustain them. Sides, it's hot and you need heat to create your abilities, so being able to channel a lot of the right heat from elsewhere instead of creating it would work wonders. The nightstalker mission is a bit more difficult, unlike in the other two, you're deep in enemy territory. I assume it's tevis's remaining light channelling into you through you taking what's left of his power when you find him. This sudden increase in light is probably the cause of it. It's possible that a similar effect goes on in the tower and in mayhem crucible. But idk. We've never had to deal with enemies there.