[Skyver]

If I had to vote on the currently suggested rules I would go with the ones suggested by @lefton4ya and @Astroking112 . I think if we are involving the d20 that is the correct way to go. I like the way you add the character's height. I involved height in a different way, as you will see below.

I like the idea of quicksand being mostly harmless, yet sometimes deadly. I also think it should affect anyone passing through, rather than only when you begin your turn on it. Moving through quicksand should be a risk/reward based gamble.

Here's my take on it:

Quicksand Terrain Rules:

ENSNARE:
When a unit tries to leave or ends their turn on a quicksand space, roll a single combat die. If you roll a Skull, that units gains a Sand marker*. Repeat this process until no skull is rolled. Before moving out of a Quicksand space, your unit must spend Movement to remove that many Sand markers. As long as a unit has a Sand marker, it cannot move away and cannot attack non-adjacent figures. Quicksand tiles counts as Sand spaces for the purpose of special powers.

BURY:
If the number of sand markers on a unit are equal to or greater than its Height value, and that unit has no more Movement to spend, that figure is destroyed. Soulborgs, Elementals, and Undead are unaffected by this ability.

*Note: Sand markers are similar to wound markers. This means that Hero figures can put markers on their Army card, while Squad figures should put the markers by their bases. Markers can be represented by dice or another indicator, as agreed upon by the players.

...

Other Rules:

Spoiler Alert!

For more deadly scenarios:

Hostile Quicksand:
Increase the Sand markers to 2 per skull rolled.

Ferocious Quicksand:
Increase the Sand markers to 2 per skull or shield rolled.
Re-roll on either a Skull or a Shield.

As an additional rule, and specifically for the more deadly scenarios, it may be a cool and thematic addition to allow friendly units to aid figures caught in Quicksand.

HELPING HAND:
Instead of attacking, a friendly unit adjacent to a figure with Sand markers may choose to remove up to 2 Sand markers from the affected figure. Figures with Sand markers may not activate this ability.

I broke up my Ability into parts for readability, but if you want a Summary version I went ahead and added that here:

Spoiler Alert!

Quicksand Summary:
When a unit tries to leave or ends their turn on a quicksand space, roll a single combat die. If you roll a Skull, that unit gains a Sand marker. Repeat this process until no skull is rolled. Before moving out of a Quicksand space, your unit must spend Movement to remove that many Sand markers. As long as a unit has a Sand marker, it cannot move away and cannot attack non-adjacent figures. If the number of Sand markers on a non-Soulborg, non-Elemental, Non-Undead unit is equal to or greater than its Height value, that figure is destroyed. Instead of attacking, an adjacent friendly unit without Sand markers may choose to remove up to 2 Sand markers from an affected figure. .