Not much is known about the history of the Zoldyck family other than they have been around a very long time. Currently there have been shown family members from 4 generations. With the grandson Killua Zoldick being one of the main protagonist in the series. Zeno the grand father of Killua was one of the family members said to have fought Netero. Although he admits Netero was old when he was still a baby. Silva the father of Killua at some point in the past as well was handed a contract for Phantom Troupe Member #8 while he succeeded in killing the target this lead to a fight with the Troupe's Leader Chrollo
The Zoldyck family is very rich. Owning Kukuroo Mountain and the surrounding area. This protects the family's privacy and makes it difficult for intruders. The gate has a security booth at the front that's mainly a decoy for those trying to force entry. The guard is to give the aggressive party a key to the false door way. Upon entering intruders will be dealt with by Mike. Intruders that make it beyond this point find themselves lost in the woods and having to deal with the butlers. Inside the gate there is also a small building to house the gate keepers.
The estate seems specifically designed as a training ground and fortress. Even though the Zoldycks are well known there's almost no records showing what they look like. A picture is able to be sold for millions of even one family member.
The Zoldyck Mansion at the back of the estate on the mountain is a large Victorian style house on the outside. On the inside it has been shown to house rooms for each of the family members. As well as rooms for training , torture, a meeting room, the butlers main office and a special room for Alluka.
As the Zoldyck family is very famous. There are security measures for intruders and nosey interlopers alike. The first of these is the False gate and the testing gate. The false gate is entered by those who threaten the gate keeper and are given a key. On the other side of the gate before reaching the forsest is Mike. A trained killing dog the size of a tour bus that devours those foolish enough to come through that gate. It has been stated that Mike will likely attack even those he knows who come through that gate. The Testing gate is actually 8 gates nestled inside eachother. The first one is 2 tones, each successive gate doubles the weight of the first gate. Killua was able to oben gates 1-4 at the series start, while Gon with the help of Kurapika and Leorio opened one through 3.
If Mike is away or not able to deal with intruders for some reason. The next line of defense falls to the Butlers. The Butlers are highly trained martial artist and Nen users. Almost as dangerous as the family they serve they are drilled within an an academy within the mansion to become very loyal very lethal servants of the Zoldycks.
The Zoldycks are among the most powerful Nen users in the world. With abilities ranging from super strength, speed, shape changing, energy manipulation etc. They so far seem to be one of the three most powerful groups of Nen users in the world. (The other two being the Phantom Troupe and Hunters Association respectively)
Part of their exceptional abilities comes from their training. From a very young age all children are trained in the arts of assassination. This includes everything from physical and mental training to learning how to resist torture. At a certain age all children are to choose a specialization for their assassination techniques.
Sometimes family members will end up on opposing sides of the same job, or at odds over an internal or external objective.
In these moments there are rules that must be followed.
1) Family members are not allowed to kill eachother. However it should be noted it looks like they can bring in an outside party to attempt to kill the other member. However given the families abilities this seems unlikely to work.
2) Family members can be restricted in their movements by the current head of the family. This means if there is perceived danger to the family or family secrets the Head (Currently Silva) can put restrictions on the offending family member. These include monitored and filtered communications , as well as how close they have to stay to an objective. Breaking restrictions will have the family member subdued by a squad of butlers and returned to face punishment.
3) There are no secrets when it comes to work between family members. Family members can take contracts for opposing factions (as seen in the Phantom Troupe arc). This does not cancel out either sides contract as long as the other side is still alive. Contracts are terminated if the client dies before hand with no specific agreement to carry on after they die.
4) Never break your word on a deal. While the details of consequences aren't mentioned. Killua does tell Gon during greed island that even though his brother hates him he would never risk breaking his word on a deal.
These are the currently known family members:
Killua Zoldyck - One of the Main protagonist of the series. His growth increases when training and working with Gon. Killua starts off stronger than Gon. But the gap seemed to be closing
Zeno Zoldyck - Zeno is the grandfather of Killua, and father of Silva. He has been seen to be a very powerful Nen user and was also friends with Netero.
Silva Zoldyck - Silva is Killua's father and current head of the Zoldyck family. He has been shown to be immensely strong, resilient and a powerful nen user, holding his own against and even killing a Phantom Troupe member.
Illumi Zoldyck - Killua's older brother and one of the series antagonist. He is very powerful and has blood lust when angered that even makes Hisoka take notice.
Kalluto Zoldyck - Kalluto is one of Kulla's younger sisters. Currently the number 4 of the phantom troupe. Kalluto is learning she isn't as powerful as she thought she was but will endure and become stronger.
Alluka Zoldyck - the youngest child and least trained. Alluka is potentially one of the deadliest family members. Sharing a body with nanika (something) so far something's powers have yet to shower a limit.
Kikiyo Zoldyck - Silva's wife and mother to all the current children. She is a nen manipulater. While her abilities have yet to be shown it's proven she's powerful by easily dispatching a butler with a thrown object and surviving being stabbed in the head bother with no effort.
Milluki Zoldyck - Killua's brother this one appears overweight and prefers to kill from afar with various intelligent sounding techniques. He also enjoys torture and is the tech expert for the family.
Maha Zoldyck - Grandfather of (supposedly) Zeno Maha is the oldest assassin of the Zoldyck family.
Zzigg Zoldyck - Another past family member Zzigg supposedly went on the first Dark Continent expedition with Netero.