Ruddy

Hello, I am confused about the way Halo of Blood artefact of power works... Does it allow the bearer to activate at the start of combat, and then, if it is my turn, allow me to activate a different unit? Or does it mean that the bearer always activates first, and then the opponent goes, no matter what turn it is. I keep reading people throw the word "basically", but it doesnt explain much... Last night, my Bloodthrister had the artefact. he activated first, on my turn, but then opponent said it was his turn to activate. Could someone explain? Thank you!

Where in the Blades of Khorne Battletome does it say that the maximum is 8 Blood Tithe Points? Also, if I want to summon anything else, like a unit of 10 bloodletters, do they need to be included in the army roster before the match?