the higher charsima the higher dmg for bard

Save DCs are 8proficiencystat modifier; this using charisma bonus of 1 is would be plausible and explain the DC of 11.

Every humanoid and giant within 300 feet of the harpy that can hear the song must succeed on a DC 11 Wisdom saving throw or be charmed until the song ends.

And so on with creatures like Medusas (gaze), Dryads (charms) and Angels (spell-like abilities) with ability score changes.

I do not do it anymore for my own games since it takes time but does not add any value.

If you want to build the harpy bard like a player character, it seems reasonable to assume that the luring song is a race feature.

In this case you could adopt a calculation like for a dragon born breath weapon, i.e.

If you just want to be consistent with the MM, the same calculation method is also good.

Pushing, pulling and knocking prone uses str, charm and fright use charisma.

Some abilities of spellcasters use their spellcasting ability.

Almost everything else uses con, including breath weapons and poisons.

The solar and the gas spore have DCs which seem to be set differently.

Some, e.g. the dragon turtle and demilich seem miscalculated.

Still, really enjoyed your opener: DMs are aiming for Player Character experience - and not so much the making of nice alignment of numbers.

The other option is to calculate a DC by using 8 prof appropriate ability modifier.

DMG, 278).

The save DCs to resist such effects have a direct bearing on the monsters challenge rating, and vice versa.

Use the Table. You can start with the monsters expected challenge rating and use the Monster Statistics by Challenge Rating table to determine an appropriate save DC for any effect that requires a tar e to make a saving throw.

Calculate the DCs.

Alternatively, you can calculate a monsters save DCs as follows: 8 the monsters proficiency bonus the monsters relevant ability modifier.

For example, if the effect is a poison, the relevant ability is probably the monsters Constitution.

If the effect is similar to that of a spell, the relevant ability might be the monsters Intelligence, Wisdom, or Charisma.

A DC of 11 aligns with the harpy having a proficiency bonus of 2 (CR 1 monster) and a Charisma modifier of 1.

The same section specifically allows for DMs to adjust stats and modifiers to complete their vision of the monster.

DMG, 274).

While the DC from the harpys Luring Song may have been initially calculated based on the creatures Charisma bonus, theres no necessary connection between them.

But it doesnt say that changes in those abilities during play will affect the monsters traits.

Some monsters do explicitly have spellcasting traits which define a spellcasting ability, so in those cases there is a link between the ability score and the monsters stats.

The DMG has some guidelines in Chapter 9, which include deriving things like save DCs from ability scores.

In 5e, monsters are simulated first then a level is determined (and then it may iterate); in 4e, a level and role is determined first then the monsters stats are determined.

The Higher Charsima The Higher Dmg For Bard