With the grounders there has always been a legend around them saying that one commander will find there soulmate and be the only commander to have a child after lexa and Clarke slept together and lexa died they didn't know that clarke is carrying lexas baby so when Clarke when you the city of light becca revealed to Clarke that she is holding the future in her body and that because of how important the child will be that when Clarke returns lexa will as well and they both had to have the flames in them so becca told Clarke when she wake to not remove them flame and that now she had black blood in her body as well as the baby that she was safe