When you are summoning a new pet, right when it appears it'll have partial health and mana but after a split second the pet health/mana bar jumps to full. If you click dark pact immediately after summoning, you'll gain the mana from your pet then his health and mana bar will immediately jump to full. Basically by doing this you can effectively lower the mana cost of summoning any pet you use.
PS - Searched for a similar topic and clicked the 'Check if Already Posted' button with no results
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