"onlar igiddir" and "onlar igiddirlər" are correct. -dır, -dir, -dur, -dür parts that attach to nouns and adjectives form verbs, english alternative would be "is/are", for example: He is a doctor. - O həkimdir. She is nice. - O gözəldir. He is brave - O igiddir. Therefore, "onlar igidlər" is not correct, in translation it means - they brave ("are" is absent). And "igidlər" is just a noun in plural form