Testamentet viser testamentsarvingen.

Because testament is a neuter noun in Norwegian: et testament.
When a neuter noun is definite singular, it usually takes -et:

• et testament = a will
• testamentet = the will

So -et here is the definite ending, not a separate word.

The base noun is arving, which is a common-gender noun:

• en arving = an heir
• arvingen = the heir

The full word testamentsarving is a compound noun, and the last part usually decides the gender of the whole compound. Since arving is common gender, the definite singular form becomes testamentsarvingen.

That s is a linking s, which is very common in Norwegian compound nouns.

So the word is built like this:

• testament
• • linking s
• • arving

This gives testamentsarving, literally something like will-heir or testament-heir.

Not every compound uses a linking s, but many do, and this is something learners usually just have to get used to word by word.

Yes. In Norwegian, compound nouns are normally written as one word.

So:

• testamentsarving is correct
• testament arving is not correct as a normal compound spelling

This is different from English, where similar expressions are often written as two words.

In this sentence, Norwegian uses normal subject–verb–object word order:

• Testamentet = subject
• viser = verb
• testamentsarvingen = object

So Testamentet viser testamentsarvingen is understood as The will shows the heir, not the other way around.

Because Norwegian has very little case marking on nouns, word order is very important.

Viser is the present tense of å vise.

• å vise = to show
• viser = shows / is showing

In a sentence like this, the present tense often has a general or factual meaning, not necessarily something happening right now at this exact moment.

Because Norwegian usually puts definiteness at the end of the noun instead of using a separate article the way English does.

So:

• testamentet = the will
• testamentsarvingen = the heir

This is one of the biggest differences between English and Norwegian noun phrases.

A separate determiner can appear in other structures, especially with adjectives, for example det store testamentet, but with plain nouns like these, the ending does the job.

It can often be understood more like indicates, states, or shows who it is, depending on context.

In legal or formal language, English show and Norwegian vise can both be used in this broader sense. So the sentence may suggest that the will identifies or makes clear who the heir is.

In some contexts, Norwegian speakers might choose a more precise verb, such as angir or use a fuller sentence like Testamentet viser hvem testamentsarvingen er.

It is grammatical and understandable, but without context it can sound a little formal or slightly unusual.

A Norwegian speaker might also say:

• Testamentet viser hvem testamentsarvingen er
• I testamentet står det hvem testamentsarvingen er
• Testamentet angir testamentsarvingen

So your sentence works, but depending on the situation, another wording may sound more natural.

They are:

• et testament = a will
• en testamentsarving = a testamentary heir / an heir under a will

So the sentence uses the definite singular forms of both nouns:

• testamentet
• testamentsarvingen

This is why both nouns end with definite suffixes.

• testament is neuter, so it takes et in the indefinite singular and -et in the definite singular.
• arving is common gender, so it takes en in the indefinite singular and -en in the definite singular.

The compound testamentsarving follows the gender of arving, the final element in the compound.

Usually, yes. Definite forms often suggest that the speaker assumes the listener can identify them from the context.

So testamentet suggests the specific will being talked about, and testamentsarvingen suggests the specific heir being referred to.

That said, in legal or formal writing, definite forms can sometimes also sound more generic or institutional than they would in everyday conversation.