Lightseed

"A single-celled organism, completely infected. Scurries about simple-mindedly. Strange air has been seeping down from above for years. Some of that air became liquid, and some of that liquid became flesh, and some of that flesh came to life. I don't know what to make of it."

Lightseeds are enemies in Hollow Knight. They are small creatures who have swollen greatly in size due to infection.

Behaviour and Tactics
Completely harmless, not even dealing contact damage. Hitting them with the Nail provides no SOUL, although they do give SOUL if they are hit with the Dream Nail.

Multitudes of them spawn before the battle with the Broken Vessel, attempting to fill it with Infection. Killing these Lightseeds does not decrease the numbers required for the Hunter's Journal entry.

Location
Lightseeds are found in the area in Ancient Basin leading to Broken Vessel. After Broken Vessel is defeated, they no longer show up in this area. They are also located in the Infected Crossroads. The Lightseeds in the room east of the lift leading to the City of Tears do not show up after Broken Vessel's defeat as well. The other five Lightseeds in the Infected Crossroads do show up even after defeating Broken Vessel.

Trivia

 * They cannot be harmed by any summoned familiars such as Grimmchild or  Weaverlings.
 * In the Kickstarter campaign, there is a GIF of a Lightseed popping out of a Husk Warrior after the Knight kills it. They were likely to spawn every time an infected enemy was killed, however, this is not the case in neither the other Kickstarter campaign images nor the actual game.

References

Germen de luz Светик Świetliste Nasiono Blob Parasite