Here are the most relevant rules for this question:

509.1 First, the defending player declares blockers. This turn-based action doesn’t use the stack. To declare blockers, the defending player follows the steps below, in order. If at any point during the declaration of blockers, the defending player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration (see rule 721, “Handling Illegal Actions”).

509.4 Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)

In Magic: The Gathering, unless you have priority, you can't cast spells or activate abilities (aside from mana abilities). Since you don't get priority until after blockers are declared and damage assignment order is determined (second for attackers, third for blockers) it is too late to activate Harpy's ability and still block with the zombie created.

You can however still sacrifice the Dissenter to the Harpy after blockers are declared. The Child of Night will still be blocked, dealing no damage and getting no life link, and the Harpy will still be a 3/4, large enough to kill the Responder. You will have to take the hit from the Sun Sentinel there, unless you have other creatures to block with or other ways to remove it from combat.

Here are the most relevant rules for this question:

509.1 First, the defending player declares blockers. This turn-based action doesn’t use the stack. To declare blockers, the defending player follows the steps below, in order. If at any point during the declaration of blockers, the defending player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration (see rule 721, “Handling Illegal Actions”).

509.4 Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)

In Magic: The Gathering, unless you have priority, you can't cast spells or activate abilities (aside from mana abilities). Since once the declare blockers step has started you don't get priority until after blockers are declared and damage assignment order is determined (second for attackers, third for blockers) it is too late to activate Harpy's ability and still block with the zombie created.

You can however still sacrifice the Dissenter to the Harpy after blockers are declared. The Child of Night will still be blocked, dealing no damage and getting no life link, and the Harpy will still be a 3/4, large enough to kill the Responder. You will have to take the hit from the Sun Sentinel there, unless you have other creatures to block with or other ways to remove it from combat.

Here are the most relevant rules for this question:

509.1 First, the defending player declares blockers. This turn-based action doesn’t use the stack. To declare blockers, the defending player follows the steps below, in order. If at any point during the declaration of blockers, the defending player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration (see rule 721, “Handling Illegal Actions”).

 

509.4 Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)

In Magic: The Gathering, unless you have priority, you can't cast spells or activate abilities (aside from mana abilities). Since you don't get priority until after blockers are declared and damage assignment order is determined (second for attackers, third for blockers) it is too late to activate Harpy's ability and still block with the zombie created.

You can however still sacrifice the Dissenter to the Harpy after blockers are declared. The Child of Night will still be blocked, dealing no damage and getting no life link, and the Harpy will still be a 3/4, large enough to kill the Responder. You will have to take the hit from the Sun Sentinel there, unless you have other creatures to block with or other ways to remove it from combat.

Here are the most relevant rules for this question:

509.1 First, the defending player declares blockers. This turn-based action doesn’t use the stack. To declare blockers, the defending player follows the steps below, in order. If at any point during the declaration of blockers, the defending player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration (see rule 721, “Handling Illegal Actions”).

509.4 Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)

In Magic: The Gathering, unless you have priority, you can't cast spells or activate abilities (aside from mana abilities). Since you don't get priority until after blockers are declared and damage assignment order is determined (second for attackers, third for blockers) it is too late to activate Harpy's ability and still block with the zombie created.

You can however still sacrifice the Dissenter to the Harpy after blockers are declared. The Child of Night will still be blocked, dealing no damage and getting no life link, and the Harpy will still be a 3/4, large enough to kill the Responder. You will have to take the hit from the Sun Sentinel there, unless you have other creatures to block with or other ways to remove it from combat.