I’ve noticed in a YouTube video that a guy had full diamond hearts. I don’t know if it was a temporary potion or if he used something to permanently increase his health. I mean after using life crystals and life fruits.

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2 Answers

This guy in your video was using the Calamity Mod. You'll have to download the mod before you can have diamond hearts. (was it Chippy?)

To increase your max HP in base terraria, you have 3 options:

1. Find a Life Heart underground, and use it. Increases max HP by 20, to 400 cap.

2. Find a Life Fruit in the Underground Jungle, post-Plantera (or anytime on mobile, old-gen console, and 3DS). Increases max HP by 5, 500 cap.

3. Use a Lifeforce potion. This is a temporary potion that increases max HP by 20%.

Edit: About the diamond hearts:

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You can increase your health with Life Crystals, found anytime underground, and Life Fruit (Starts growing anytime on Old Gen console and mobile. Only spawn after one Mechanical Boss has been defeated on Desktop and Console). After that you've reached the highest health you can be at in vanilla Terraria without the Lifeforce Potion, namely 500. There are many mods that can be used to increase your health though.