Bind Demon

The Smith from Act 1 is also a great target early on once you have access to the skill

Bind Demon is actually a nice way to have a closer look at the monsters' graphics. Usually they die so quickly, that you can hardly take a look at them.

Respeccing your character will kill the bound demon, even if you have a Dagger or book equipped with the ' Bind Demon' skill.

Having a sundering charm in your inventory will sunder your bound demons immunities as well.

Ever wanted to run your own day-care centre?

Well now you can!

Bind a Vile Mother, and they will create up to 5 Vile Children before they start attacking enemies.

No having to fuss around looking for Fallen to Revive either.

On another note, any children that are born in a Terror zone will inherit that zone's Enchantment (very noticeable with extra fast)

Based on specific testing, at least some of the stats (e.g. affixes) of a bound demon are locked in at the time of binding and will not change with new games, even if you equip/unequip +skill gear or assign further hard skill points into related skills.

As such, while leveling, if you improve your related skill points, you may want to look at finding a new demon afterwards as your current one seemingly won't necessarily receive all of the additional benefits retroactively.

Similarly, there is an argument to be made for specific demon-bind gear, focusing on related +skills to make your bound demon as good as you can make it as you can unequip that gear afterwards without losing the benefits on your bound demon (or at least not all of them).

There are some additional caveats that other commenters have not mentioned:

• Fetish Shaman (except Witch Doctor Endugu) can Revive nearby Fetish, similar to Fallen Shaman.
• Bishibosh can not Revive nearby Fallen.
• Creeping Feature (in Stony Tomb Level 2), despite being Undead, is able to be bound with this skill.
• You can bind other players' summons from Summon Goatman, Summon Tainted and Summon Defiler if you declare them as Hostile. When used on a Goatman, its Crushing Blow is based on your points placed in Death Mark.

Due to how resistances work on monsters, if a bound demons resistance goes over 100% due to auras, they become immune to that damage type.

This also bypasses the rule that prevents monsters from gaining more than 2 immunities from enchantments.

Binding a demon with the Cursed Affix insanely boosts DPS for builds that deal physical damage, like the popular Echoing Strike. It is more important than finding one with a Fanaticism aura.
I've been running around players 8 with a cursed Hephasto with Conviction (which doesn't help the Echoing Strike build) and Gull for a weapon instead of my standard Arioc's Needle and I can still clear most areas very efficiently, but with a 100% boost to MF. As soon as the Amplify Damage curse hits, monsters start to melt.

Hephasto is a popular choice for it's availability, but in my experience you'll need some tries before finding a cursed one. But any demon with the cursed affix will do and there are a lot of Demons on the council on Travincal that might be cursed. It may be faster to Farm Cursed Demons by going Hephasto then Travincal in each run. Then, if none are cursed, repeat. You can also include other Act 4 areas if you want as they contain a lot of demons.

I think a cursed Council Member is good enough, but Hephasto is better because he only attacks melee, which helps redirect damage from act 2 mercenaries, while the council members sometimes wander around and let the merc (or you) take the hits. But you can use the (in my experience) easier to find cursed Council Member while you farm Hephasto/Act 4 or just do any other farming. Just remember to use Blade Warp/ Teleport frequently to position your Merc in a way he doesn't die.

It is said that farming for a Cursed + Fanaticism Hephasto is ideal, but it can take a good amount of time to find this one specifically. And considering you lose the bound demon when you die, it's good to find quicker alternatives. In singleplayer, resetting your maps to have a short path between a waypoing and The Hellforge is great for a non HC Warlock.

Be careful, if you accidently bind a new demon, your old one (which might have a precious aura) will die. So it is NOT a good idea to assign Bind Demon to a hotkey.

On Nightmare, with level 10 Bind I bound a Carver Shaman who revived up to five Carvers, and then a Stygian Hag ( Vile Mother) who summoned up to five of the little dudes they summon. That seems to be the limit.

Despite normally being immune to curses, a bound possessed champion can still be cursed.

This Dark Lancer lady hits like a truck.

A Warlock can only have 3 demons? Not if he binds a Fallen Shaman

I have read on forum that if you have 1 hard point and +19 skill points to demon tree, you can remove all +19 skill points for that game and still have same damage.
but if you do not put back on the +19 skill before making next game, you may still have all the affixes, but all your damage def and health buffs from all demon tree skills will be re calculated at level 1