Legends guards are unattackable guards outside the Legends' Guild. They wear what appears to be blurite armour and carry an iron spear.
The two guards will greet you when you go through the gate. Their message varies based on your status in the Legends' Quest: during the quest they say "I hope the quest is going well [sir/madam]", while after the quest they say "Attention! Legends' guild member approaching!".
If you talk to one or try to open the gate before starting/completing the Legends' Quest, the guards will talk to you about what the Legends' Guild is and the requirements for the quest (if you do not already have them).