Does your bound demon's damage scale as you level up the skill? Or do you need to rebind a new demon each time to take advantage of hard points invested?

When you bind a demon, the following things are locked in permanently;
>monster type & level (norm/nm/hell/tz and area differences), affixes on the monster (including added from skill and TZ affixes added to normal mobs), which aura is selected for aura enchanted, its playercount HP scaling (100-450% for P1-P8), and its random name if upgraded to unique.

Every time you join a game, your demon is resummoned using your current skill levels after gear bonuses, meaning it will never include battle command / skill shrine to help, and you can swap gear before leaving a game and after joining a new one for maximum +skills
At that point it will determine for the length of each game:
>monster HP +% from blood oath, added %dr, +base damage, % enhanced damage from mastery
and only when you use skills will it gain the temporary buff of engorge, auras, etc.

I've tested some not all so I can't be 100% sure on every case, but its clear enough on damage that if you have a level 1 bind + 1 mastery and equip +98 all skills gear, the demon will 1 shot everything when resummoned at the start of the game with level 99/99 skills, but take it off and join a new game and it will revert to dealing piddly damage. So I'm reasonably sure that both the +base damage and +% enhanced damage only apply when resummoned

beautiful info.

ok so it sounds like the hard points dont matter bc it technically resets on every new game, but it would be worth if you can somehow manage to get a P8 TZ demon with all the desired buffs (cursed, aura, extra fast)

If all you want is to bind for the right aura/cursed combo you can do 1 point bind demon and stack +skills to reach 20+ so you get aura enchanted, then it'll resummon each game with that aura but at piddly low level hp/damage
If you want the highest damage you can get from him, you want to max bind & mastery

right now its the difference between about 16k damage per hit and 23k with my urdar if I switch to max +skills gear between games, but using only 1 point and low +skills you might only have 2-3k damage

i have a blood boil lock and after maxing out boil + its synergies, i have about 40ish points leftover so im debating where to put it

the only complaint i have for the build is the p8 baal damage is a bit meh so im trying to decide if maxing my bind demon and demon mastery is the way to go, or maybe i try to dump 20 into echoing strike + rest into mirrored blades and see if that helps my boss damage a bit