I really like the idea of a type of spider that weaves a web of conductive steel like substance. It is drawn to sources of lightning and normally makes its webs in the tallest trees of the tallest forests to try and catch as many lightning bolts as possible. In nature it has an egg sack in the middle of its web, the eggs get empowered with elemental power from each lightning strike, it is sort of how they feed their young. They still eat anything else, just that electricity supplies their magical abilities, which allows them to hunt.
Imagine a web of tough conductive steel strands around the lair of a blue dragon. That makes it really tough, as if you are standing in that stuff, or get caught in it, you really can't avoid a lightning attack. The spiders can zap you up close and personal, or send charge through their webs to stun you. While you are stunned they wrap you up in steel-like cable for later eating. If you are wrapped up and they decide to zap you, it is auto-crit time.
With a blast of its lightning breath, the dragon can hatch the egg sacks, releasing a swarm of smaller and highly charged spiders. This lets you use a swarm, as well as the giant spiders. They could be summoned as a lair action.