1HD: You can take fiend classes and fiend feats even if you aren't an outsider (this allows you to take a level of a fiend class at 1st level). You also stop aging before you reach middle age, and you don't die of old age. 5HD: You gain DR/Good or (choice of baneful substance) equal to your HD/3, rounded down. If you gain DR from another source, the DR stacks and is bypassed by the existing DR's weakness. 10HD: Choose one spell of 3rd level or lower that is fiend appropriate (DM needs to approve). You can cast this as a spell like ability 3 times per day as a swift action.