Core rule
High and low ground
Attacking from higher ground can improve your chance to hit; attacking from below can reduce it.
At least 2.5m above the target grants +2 to attack rolls, while the same distance below applies −2. This usually matters for ranged attacks, but melee attacks also qualify if they can reach across that elevation. These are flat modifiers, separate from Advantage and Disadvantage.
Consider the movement and resources needed to reach high ground. Spells resolved with saving throws do not use this attack-roll bonus.