Lo llamaban el diablo.

Lo is the direct object pronoun, and here it means him.

It stands for the person being called el diablo.

For example:

• Llamaban a Juan el diablo.
• Lo llamaban el diablo.

So lo replaces a Juan.

In standard Spanish, lo is the normal pronoun for a masculine singular direct object.

In this sentence, the person being called something is the direct object of llamar, so lo is the expected form.

However, in much of Spain, you may also hear:

• Le llamaban el diablo.

That is because of leísmo, which is common in Spain for masculine human direct objects. So:

• Lo llamaban el diablo = fully standard everywhere
• Le llamaban el diablo = common in Spain if the person is male

Llamaban is the imperfect indicative, 3rd person plural, of llamar.

So it comes from:

• llamar = to call

And llamaban means:

• they were calling
• they used to call
• people called

depending on context.

The imperfect is often used for:

• repeated actions in the past
• habitual actions
• background description
• ongoing situations

So Lo llamaban el diablo suggests that this was what people used to call him or that it was a label applied to him over a period of time.

Compare:

• Lo llamaban el diablo = people used to call him that / that was his usual nickname
• Lo llamaron el diablo = they called him that on a specific occasion, or they gave him that name at a definite moment

It is 3rd person plural, but Spanish often uses that form with an unspecified subject.

So llamaban can mean:

• they called him
• people called him
• everyone called him

It does not have to refer to a specific known group.

Because se llamaba and lo llamaban are not quite the same.

• Se llamaba... usually means his name was... / he was called...
• Lo llamaban... means people called him...

So:

• Se llamaba Juan = his name was Juan
• Lo llamaban el diablo = people called him the devil

In other words, se llamaba focuses more on the name itself, while lo llamaban focuses on what others called him.

The pattern is:

• llamar a alguien + noun/adjective

meaning:

• to call someone + something

Examples:

• Llamaban a Juan el diablo.
• Llamaban a Marta la reina.
• Lo llamaban loco.

So in Lo llamaban el diablo:

• lo = the person
• llamaban = called
• el diablo = the label/name used for him

Because Spanish often uses the article with a common noun used as a label or nickname-like description:

• el diablo
• la reina
• el loco

So Lo llamaban el diablo means they called him the devil.

If you said Lo llamaban Diablo, that would sound more like Diablo was being used as a more fixed proper nickname, like a name.

Because el diablo is not the direct object here.

The direct object is already lo.

The phrase el diablo is the name/label being applied to that person.

That is why:

• Lo llamaban el diablo = They called him the devil

But:

• Llamaban al diablo = They called/summoned the devil

Those are two different structures with two different meanings.

Yes. Lowercase is normal here because diablo is a common noun.

So:

• el diablo = the devil

In many contexts, this is a figurative nickname or description, not necessarily the literal Devil.

If it were being treated as a formal alias or stylized nickname, you might sometimes see Diablo or el Diablo, but el diablo in lowercase is the normal default.