MAKING A SAVING THROW
To make a saving throw, roll a D20 and add the appropriate Ability Modifier for the character. Just like Ability Checks and Attack Rolls, you must hit the target number or higher in order to succeed.
A saving throw can be modified by a situational bonus or penalty and can be affected by Advantage and Disadvantage.
Saving Throw: Dexterity for sacred flame
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Description: Curse a creature to deal an additional 1d6 Necrotic whenever you hit it with an attack. The creature also has [b]disadvantage on ability checks[/b] for one ability of your choosing.
Disadvantage is a mechanic in Baldur's Gate 3. While players are under the effect of Disadvantage and perform an ability check, saving throw or an attack roll they will roll a second D20 while making that role and use the lower of those rolls to decide the outcome.
A Saving Throw is different than an Ability Check, as your 2nd line reads.
If your final line implies that in BG3, if you ever have (dis)advantage, then it applies to STs, Ability Checks,
and Attack rolls, then Larian has modified this from 5e rules-as-written.
In 5e, you can get (dis)advantage on one, two, or all 3 of those depending on the source. It's not always all 3.