This is a spoiler log for the Runic Icosahedron relic that you might find on your adventures. Let me know if I'm missing any effects for Natural 20 or Natural 1 as I was pretty far into a run before attempting to test every number.

Explanation of Randomness with Relic
Runic Icosahedron relic is truly random every time you right-click it in combat unlike most things in Slay the Spire which are consistently random. Truly random means that it will give you a different result nearly each time. Consistently random means you will get the same result each time. While the roll is truly random, the effect of the roll is consistently random.

For instance if you roll a 16 and get an Entropic Brew, restart the battle a few times and roll another 16, you will always get an Entropic Brew. The potions you get from that Entropic Brew are also consistently random. 17 will always give you the choice of the same three random cards. 12 will always give you the same potion but not the same potion as 16.

Roll Results
20: +Gain [E] at the start of each turn. [Permanent effect.]
+Gain 1 additional [E] when rolled, this turn.
+Can't roll the die anymore this run.
19: +Heal to Full.
18: +Gain [E] at the start of each turn this combat.
17: +Choose 1 of 3 random rare cards to add to your hand this combat.
[Rare cards are limited to your class unless you have relic(s)/effect(s)
that let you discover other class/colorless cards.]
16: +Gain 1 random Rare potion. [e.g. Entropic Brew]
15: +Draw 1 additional card each turn this combat.
14: +Gain 1 Strength, 1 Dexterity, and 1 Focus this combat.
13: +Gain 3 Artifact this combat.
12: +Gain a random common potion. [e.g. Attack potion]
11: +Gain 100 gold.
10: +Add a Mulligan card to your hand this combat.
[The Mulligan card allows you to discard your hand
and redraw that many cards once per combat. Keyword: Purge]
9: +Lose 50 gold.
8: +Gain 5 Weak this combat.
7: +Gain 5 Frail this combat.
6: +Gain 5 Vulnerable this combat.
5: +Increase the cost of all cards in hand by 1 this combat.
4: +Draw 1 less card each turn this combat.
3: +Add 3 Void status cards to your draw pile this combat.
2: +Lose 10 HP.
1: +Add 1 soulbound curse to your deck that removes 1 [E] when drawn.
+Adds curse to hand when rolled.
+Can't roll the die anymore this run.