Situational AP purchase
When should you buy anti-heal?
Buy healing reduction when enemy healing or regeneration is deciding fights and you can keep damaging the relevant target. Grievous Wounds reduces healing; it does not reduce shields, magic damage or crowd control.
Start with the component
Oblivion Orb can provide healing reduction without immediately paying for a completed Morellonomicon. Fit the component around your core damage, mana and survival needs; check the current queue's item tooltip.
Finish when it fits the fight
Complete Morellonomicon when its stats and slot efficiency suit your build. Do not treat it as a mandatory first item just because your champion deals magic damage.
Coordinate coverage
Multiple Grievous Wounds sources do not add their percentages together. Another reliable applier may cover the same target, but you may still need your own source for a separate fight or target.
Repeated spell damage and damage over time can maintain coverage more reliably than a single hit. Match the purchase to your actual spell access, enemy sustain and the teammates who can apply the debuff.
Will the fight last long enough?
If you die before dealing damage, you apply no healing reduction. If your target dies before healing, there is little healing to prevent. The debuff applies on qualifying damage immediately; it does not need to build up. It remains briefly after your last hit, so teammates can benefit even after you disengage.
Only healing during the active debuff counts. For example, 40% reduction prevents 80 of a 200-point heal, or 400 of a 1,000-point heal, before other modifiers. Compare that with the damage, penetration or survival you delayed; neither example guarantees a kill. Healing after the debuff expires is unaffected.
See the Oblivion Orb and Morellonomicon references for current effects and queue availability. These are situational decisions, not a measured best-build claim.