"unless they have a more Xtlized SOUL ... and that the reptilians DO NOT HAVE.. since they steal souls.."


Right here I'll have to call you out on not knowing too much about this. Immortals do not come from, nor are they universally involved in your planet X conspiracy theories.


As for the achilles heal, I can only answer to this if I know exactly what you mean. In myth achilles was unkillable unless through his heal, but the more realistic version that the myth was based on was probably that achilles was simply a -very- skilled fighter by nature, and somewhere along the line his heal took damage. That would not have been what killed him unless it cut an artery or an arrow/blade was poisoned, which are possibilities. Achilles could have been immortal, it would make sense of the great fighting ability and being claimed friend/son of gods, but there isn't much documented (myth) information to say that he was.

Immortals just same, most don't need an achilles heal to die.. unless you refer to common human weakness, a specific weakness.. such as forgetting to back guard whebn struck from the side. This would be lack of practice & skill.