Advancement: -

[NontheistCleric]

There are some creatures with this in their statblock. However, some of these creatures are also intelligent. One example might be the Banedead from Monsters of Faerun. My question is, does having - in Advancement stop such a creature from taking class levels even if it would otherwise be able to?

[Inevitability]

Based on several examples straight out the MM, I think the advancement line is meant to be a suggestion at most.

[RoboEmperor]

There are some creatures with this in their statblock. However, some of these creatures are also intelligent. One example might be the Banedead from Monsters of Faerun. My question is, does having - in Advancement stop such a creature from taking class levels even if it would otherwise be able to?

No. It stops them from taking racial levels. So they can get class levels, but not monster levels, because that's the end of the line for their "monster class".

[KillianHawkeye]

There are monsters that are noted as advancing "by character class", usually ones who don't have racial Hit Dice. Other monsters can take classes if they're Intelligent enough (i.e., smarter than animals), but I would consider any creature that's ignoring its typical advancement method to be a rare exception to the norm.

[RoboEmperor]

There are monsters that are noted as advancing "by character class", usually ones who don't have racial Hit Dice. Other monsters can take classes if they're Intelligent enough (i.e., smarter than animals), but I would consider any creature that's ignoring its typical advancement method to be a rare exception to the norm.

Yeah the DMG says monsters with class levels are super exceptional and very powerful.