Whenever a Swamp enters the battlefield under your control, choose one —
• You draw a card and you lose 1 life.
• Dread Presence deals 2 damage to any target and you gain 2 life.
A Swamp is a land with the subtype Swamp, not any land with a mana ability that produces black mana.
If you choose the second mode and the chosen target is an illegal target by the time Dread Presence’s triggered ability tries to resolve, the ability doesn’t resolve. You won’t gain 2 life, and the ability won’t change to the first mode.