Suggestion is a mind spell. It is low-mid level. It can only target a single person within hearing range. To cast the spell the caster must say the suggestion in a language the target can understand. Sign language can work, in which case the distance is around. The suggestion can have a trigger (such as: the next time you see the king) but the spells effect wears off when the target falls asleep.
Suggestion works by convincing the target to do something that they already might have done, or had at least considered doing.
Suggestion can’t make a person do something they are unwilling to do, like kill their family or jump off a cliff. It only works if they might have already done it.
Using a modern example: A disgruntled employee can be made to steal from their company, sabotage a production line, or even kill someone if they’ve already thought strongly about doing so. However, a happy employee couldn’t be made to do any of these things.
Suggestion can be used to trigger intrusive thoughts, but the suggestion must be cast very close to the intrusive thought popping up, and the target must have at least some inclination towards performing the suggested action. For example: a person is very depressed, driving, and thinks they should just drive into oncoming traffic and end it. Suggestion could push them over the edge and make them act on their intrusive thought, even though they normally wouldn’t do that. Not all intrusive thoughts can be triggered like this though.
In some cases, a person might seek to have a suggestion cast on them. It can be useful to overcome anxiety, fear, or laziness.