not sure about specific Monaco taxes/estate taxes but if similar to France, estate taxes are the bear...

Inheritance tax in civil code countries certainly seems very strange if you come from a common law background. The underlying logic seems to be that assets, like income, are a thing that morally belong to a household, not a person. Income tax in France is spousally joint, not individual. Since the notion is that the assets are already (in effect) collectively owned by right, the French government tells you how much of your estate you have to leave to each relative, with only a remainder you get to say anything about. (working from memory, if the number of children plus current spouse is N, you have to leave assets worth 1/(N+1) of your estate to each of them, and you get to pick where the last (1/N+1) goes).

Monaco is very mild by comparison. There is an estate tax, but only on assets you have in Monaco. The rate is a sliding scale based on degree of relationship: zero for spouse and kids ranging up to around 16% for wholly unrelated heirs. This is wildly attractive compared to the UK, though not as attractive as the relatively few rich places without inheritance/gift taxes. Canada, New Zealand, Australia, Austria, Sweden...

Of course the country of your citizenship might also levy some taxes. And, unknown to almost everybody, the taxes potentially owed to the domicile of the asset. A non-US-resident owning US stocks or property at death is subject to US inheritance tax on all of those assets, without the de minimis exemption threshold. Since the US doesn't generally know you've kicked the bucket, and non-US banks/brokers don't care, almost nobody actually pays this. If you like not to pay this but like to follow rules, it is best sidestepped simply by holding the assets via a holding company...since the company is the titular holder and a company can't die, there is no inheritance tax levied. Typically one would use a company in a jurisdiction without corporate income tax like BVI, though the paperwork is a lot more than it used to be.

Jim