The Immortal Clan is a group of people, like the name suggests, who are immortal. Their ability to live for hundreds of years suggests that many clan members still exist but so far, Free is the only notable known member to have been present through out the entire Soul Eater series.
Being part of the immortal clan appears to grant the person in question an immortal body, and even significant wounds which are inflicted to the body last mere moments. The wounds may be temporary, but Free is still capable of feeling pain. However, it is uncertain as to whether or not this is a tribute of the entire clan, as Free is the only known member, meaning that such an ability may be unique to solely the werewolf bloodline. Maka speculated that her Witch Hunter might be able to purify/kill Free, due to its use of her anti-demon Wavelength. To an extent, this appears to be true, as she was capable of using it to cut off Free's tail and it has yet to grow back, suggesting the damage may be permanent. However, all other wounds caused by her Witch Hunter did heal as though they were regular injuries.
In the Video-game, Monotone Princess, a Immortal Wereworlf similar to Free, Alone, appears. He's killed by Maka when he's hit with Maka's Witch Hunter, showing techniques similar to that of the Witch Hunter are able to kill an Immortal. However it is still unsure if Alone died because of Witch Hunter, because he already had lost his Immortality by eat an innocent soul.
- There has been some controversy over whether Mosquito is a member of this particular clan, due largely to contradictory statements made about him, with Yumi Azusa referring to him as an "aging monster" within Chapter 27 of the manga but Giriko instead claiming that he is a "senile immortal" at the beginning of episode 37 in the anime. Although other evidence has been presented for either side of this argument, as no defining resolution over this issue has been made, it has consequently prevented his inclusion from the clan thus far.
- It may be possible the Immortal Clan is either hidden or mostly dead. A possible theory is that there's attacks that exist that are similar to that of the Scythe Meister Hunting techniques (Witch-Hunt , Majin Hunt , Kishin Hunt) could be responsible in their ending. Or at least why they're hidden, despite their immortality making things difficult.
- Death the Kid's comment suggest that Immortals have shown up in the past, as he was able to tell that the most effective way of fighting them off is with range attacks rather then Close Quarters Combat.