Sons of Antaeus
The Sons of Antaeus are a loyalist chapter of Space Marines who lead a highly mysterious existence.
They are known to be particularly hardy and tough, being far more impervious to damage then even other Space Marines. This is due to the mechanicus going crazy adding all sorts of extra stuff to harden them up. Due to this, their detractors having pointed out how similarly hardy they are to the Death Guard traitor legion.
Perhaps due to this, they are notably not open to most imperial authorities and avoid contact with them in what ways they can. This is more then likely a sensible choice.
Over a decade ago, these guys had their own rules in 3rd edition, found here (handily /tg/ updated for 6th edition)