The following are rules for a multi-player poker-like card game in a D20 system, Pathfinder 2E specifically.

The design goal is to be resolvable using the core mechanics (skills, proficiency, levels of success) as well as system abilities (feats and spells) that effect such checks.

Specifically, abilities such as the following:

• The skill feat Lie to Me: Use your Deception DC (instead of Perception) on Lie attempts against you.
• The divine cantrip Read the Air: +1 status bonus to Diplomacy to Make an Impression.
• The occult spell Ill Omen: cause misfortune (disadvantage) on attack/skill checks for 1 round.
• The divide/occult cantrip Wash Your Luck: cancel a misfortune effect.
• The Agadaz (Gambling Devil)'s abilities:
• Double Down: trade a -2 status penalty on a check now for a +2 status bonus on the next check, and

• Risk Aura: basically Ill Omen as an Aura effect, but with a provision that the misforutune effect caused will always take a result of a natural 20 during the disadvantaged roll.

• The Gambling Companion Tengu's ability Gamer's Guidance: advantage instead of a status bonus on successful Aid.

The Game

The game has 5 phases.

• Draw
• Bet
• Improve
• Bet
• Resolve

Phase 1 : Draw

Drawing is a roll of 3 d20s in a sort of "exploding" fashion, according to the following table:

| *roll*   |   *prob* | *result*         | *improvement*   | *L5* | *L7* |
|----------+----------+------------------+-----------------+------+------|
| 1        |       5% | no pair          |                 |      |      |
| 2-5      |      20% | one pair         | incr easy (-10) |   10 |   13 |
| 6-12     |      35% | two pair         | very easy (-5)  |   15 |   18 |
| 13-16    |      20% | three of a kind  | easy (-2)       |   18 |   21 |
| 17-18    |      10% | straight         | DC@level (±0)   |   20 |   23 |
| 19       |       5% | flush            | hard (+2)       |   22 |   25 |
| 20       |       5% | (…)              |                 |      |      |
|----------+----------+------------------+-----------------+------+------|
| - 1-15   |       5% | (flush)          |                 |      |      |
| - 16     |  5% x 5% | full house       | very hard (+5)  |   25 |   28 |
| - 17     |  5% × 5% | four of a kind   | incr hard (+10) |   30 |   33 |
| - 19     |  5% × 5% | straight flush   |                 |      |      |
| - 20     |          | (…)              |                 |      |      |
|----------+----------+------------------+-----------------+------+------|
| . - 1-19 |          | (straight flush) |                 |      |      |
| . - 20   | 5%² × 5% | royal flush      |                 |      |      |

(A previous design iteration had a 5-tier table that hewed very closely to the actual probabilities of drawing a given hand-category from a freshly-shuffled deck. That also means that there's a 50% chance of drawing "no pair", and a 90% chance of drawing "no pair" or "one pair", which is sort of boring at the (role-playing) table. We want most hands to have a chance of mattering. This table is more game-y.)

Drawing is subject to d20-influencing fortune effects (? only on the first die ?). 3 dice are always rolled, with any "unused" results retained in "reserve" for potential tie-breaking (if you roll 3 20s, you've flopped a royal flush and won't need a reserve to tie-break. :).

Phase 2 : Bet

Once a hand is drawn, Betting occurs in the usual fashion of rounds of raising, calling/checking, and perhaps folding, starting from the "left" of the dealer. While role-play is important here, mechanically characters are most likely using Lie (Deception) and Make a Request (Diplomacy) to resolve how NPCs are swayed during the Bet phase.

Phase 3 : Improve

After the betting has settled, characters can (optionally) attempt to Improve their hand. The DC for this roll is based on a by-level DC for the average level at the (gaming) table, where the DC is fixed against a Straight hand, and modified easy or hard based on "distance" from a Straight. For instance, drawing Two Pair means you have a "very easy" (-5) DC to improve your hand, where as drawing a Four of a Kind means you have a "incredibly hard" (+10) DC to improve your hand.

• critical success: improves the hand by two catgories (eg. Flush → Four of a Kind)
• a normal success: improves the hand by one category (eg. Straight → Flush)
• a normal failure: no change
• critical failure: downgrades the hand by one category (eg. Two Pair → One Pair).

The Improve roll is subject to fortune effects and other d20-check-modifying abilities.

Phase 4 : Bet

After the Improve phase is done, a second phase of Betting occurs.

Phase 5 : Resolve

Once the second round of Betting is settled, the hands are Resolved in the usual way. Any ties in the rank use the roll(s) left over from the initial Draw phase to tie-break.

For Example… the Player Alice rolls [12, 17, 5] on the Draw resulting in a Three of a Kind (the 12) with a 17 (and 5) in reserve. An Improvement-phase roll of 21 (vs. a DC 18 for 3oK for a 5th level table) is a success, upgrading the hand one category, to a Straight. During the Resolve, the character Bob also reveals a Straight, so the reserve rolls are used to tie-break. Assuming Bob rolled a "9" in reserve in their Draw phase, Alice's 17 beats Bob's 9, so Alice's (eg.) Queen-high Straight beats Bob's (eg.) 9-high Straight.

As another example, a losing player's reserved "6" might suggest a Full House consisting of 6s full of Jacks, where the winner's reserved "19" might suggest Aces full of Kings.

Colophon

This was inspired by this post.