Lamia Lore
A character knows the following information with a successful
Arcana check.
DC 20: A lamia is a bizarre fey comprised of hundreds
of black scarab beetles. It lures wayfarers to their deaths by
assuming a pleasing humanoid guise.
DC 25: When a lamia slays a humanoid creature, it adds
another beetle to its evergrowing swarm. When a lamia’s
swarm grows too large, it reproduces by first slaying a worthy
fey creature such as a powerful eladrin. Rather than consum-
ing the body, the lamia divides itself, filling the corpse with
hundreds of its beetles. Over time, these beetles devour the
corpse and arise as a new lamia. This newborn lamia gains
much of the victim’s memories and knowledge in the process.

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So, I plan on having a party encounter a Lamia who's swarm has grown too large. They have Fey in their party. It will try to outright kill one with Devouring Swarm to reproduce. There's a Drow, an Elf & a Half-Elf it could potentially pick from. If the player dies from this Lamia's attacks, my plan is to hand them a handout describing a dream they have over the next 6 hours of sleep, and arise a new Lamia. There's a scroll they can get to change back if they want to. So, I'm trying to think of what stat changes this would mean, without making these changes game-breaking. Possibilities include:

* As a swarm:
Resist takes half damage from melee and ranged attacks;
Vulnerable 10 against close and area attacks

* Change shape, similar to the Doppleganger racial power.

* They keep the same racial bonuses and powers they had before the change. (I don't want to be completely unfair.)

Any other suggestions on keeping this balanced, but having the effect of feeling more like a Lamia? Of the three possible targets, two are Rogues & one's a Cleric, if it matters.