What name is taken if a man marries into a noble house?

Something similar to what you propose was possible in the Austro-Hungarian Empire. I dont know how common it was but when my great-grandparents married, they did this. They both had titles but only the woman was going to inherit land, so the husband took her name and coat of arms after marriage.

This happened at the beginning of the 20th century in Lika, current day Croatia.

It's later than the middle ages (18th century), but the fictional Barry Lyndon (novel and film) involved Redmond Barry marrying Lady Lyndon and henceforth being known as Barry Lyndon. A main sub-plot of the story involved his struggle to get a peerage in his own right.

Thackeray based Redmond Barry on the real-life Andrew Robinson Stoney, who married Mary Eleanor Bowes, Dowager Countess of Strathmore and Kinghorne. He took the name of Andrew Robinson Stoney-Bowes.

In a fictional setting, the appropriation of a noble name by a commoner groom has some literary pedigree, and can raise some good dramatic tension in my opinion.

In one unusual circumstance, when the Count von Bohlen married Bertha Krupp (of the Krupp arms house), the man (von Bohlen) was asked by the Kaiser to add his wife's surname, Krupp, to his own. They became the Krupp von Bohlens.

This was true, even though as a member of the nobility, von Bohlen technically outranked his (commoner) wife. But the name "splicing" came about because the Krupp name had become synonymous with German armament.

First, I am assuming that you are giving your fantasy world a "Western European" flavour. Working from this assumption there are still a myriad details that vary from nation to nation within Western Europe, but in general the two houses are allied, but the offspring only marshall the coat of arms; the husband and wife are each only entitled to their own arms.

That said however, if the husband and wife are each head of their respective houses (as with Ferdinand of Aragon and Isabella of Castile), then they have authority to do otherwise than above.

You can obtain much more information on European heraldic traditions, by nationality and culture, by looking up both heraldry and the respective national Colleges of Arms on the web:

• College of Arms of England, Wales, Northern Ireland and much of the Commonwealth
• The Court of the Lord Lyon (Scotland)
• International Armourial Registry
• German Heraldry

Also note that the merging of two houses is not as simple as the wedding of respective heirs. There is usually continued friction between the nobles of the two countries, and sometimes the commoners as well. As noted in the link to Ferdinand of Aragon above, it was not until the ascension of Charles I (grandson of Ferdinand and Isabella) that Castile and Aragon truly united.

Likewise, Scotland and England still see themselves as distinct countries within the United Kingdom of Great Britain, with a common monarch, more than 400 years after the ascension to the throne of England by James the VI and I.

Note that someone who is entitled to bear (a coat of) arms may wear any arms desired, once the same have been registered with a College of Arms in order to ensure that:

• No conflict exists with arms already granted;
• No inappropriate marshaling of previously granted arms has been performed; and
• No symbols of rank (sceptres, acorns, etc.) have been used inappropriately.

Lord George, 1st Viscount Sackville, was infamous for his alleged cowardice at the 1759 Battle of Minden; and judged at court martial as "... unfit to serve His Majesty in any military Capacity whatever." Then "in 1769, Lady Elizabeth Germain died without natural heirs, and left her estates ... to him." From the following year he was then able to formally take the name Lord George Germain. By that moniker he again became infamous in Lord North's cabinet as Secretary of State for Colonial Affairs from 1775 until 1782.

At the Battle of Minden he four times ignored or refused to obey orders to advance with his cavalry, and even stopped a subordinate from doing so on his own initiative.

During the preparation of the royal wedding between The Royal Heiress to the Swedish throne and a commoner, people talked about heraldry and the possibility that a new royal house will emerge. But this changed when The Royal Household afirmed that the commener Westling will change and add his surname into The Royal Family name.