Icicle Turtle (Gargantuan N elemental) (1,100 XP)

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AC 18 HP 145 SPD 20', swim 40'
| STR +2 | DEX -2 | CON +4 | INT -2 | WIS +1 | CHA -2 |

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Saving Throws. Con +6, Str +4
Skills. Perception +3, Stealth +2
Damage Vulnerabilities fire; Immunities cold; poison Condition Immunities poisoned
Senses darkvision 60'
Languages Aquan, Auran
Amphibious. breathes air and water.
False Appearance. indistinguishable from stalagmicicles while motionless.
Ice Walk. Needs no ability check to move on icy surfaves; unhindered by ice or snow difficult terrain.
Stalagmicicles (Recharge 3-6). As a bonus action, creates 2 5x5x5 ft. stalagmicicle clumps in unoccupied squares within 60 ft. of itself. A clump has AC 13 and 4 hit points, is vulnerable to fire damage, and is difficult terrain. Deals 1d4 (2) cold damage and 1d4 (2) piercing damage to any creature that moves into its area and fails DC 14 Dexterity (Acrobatics) check, then crumbles.
Turtle start. The icicle turtle takes its turn at initiative 0.

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Bite. M W ATK +4; 1d12+2 (8) piercing, reach 15 ft.
Icicle Blast (Recharge 5–6). Each creature within 60 ft. of icicle turtle takes 2d4 (5) cold and 2d4 (5) piercing, save (DC 14 Dexterity) for none.

Icicle Turtle CR Calculation:

Offensive CR: Icicle Blast first round when it reveals itself to hit hopefully all 4 party members; 10*4=40 damage, (except it's save for none, not save for half....Average PC has +2 to DEX saves at level 4 if not proficient, +6 if they are (because it's likely their attacking stat). ~1/3 chance of being proficient, (2*2/3 +6*1/3)=3+1/3, so they pass DC 14 save with (21-14+3+1/3)/20 probability = 0.516. So, subtract 0.516*(40/2) -> Icicle Blast Effective ~30. +16 damage from the other two rounds ->46/3 = 15.3 DPR; both attack bonus and save DC within one of the expected, base offensive CR = 2. (It's still CR 2 if you don't make any adjustment for "save for none;" 18 dpr still in that bracket.) In theory assume that the stalagmicicles do a little bit of damage, but in practice they didn't do any damage, the PCs destroyed them on if the monster managed to pin them with them, which generally meant that they just counteracted the monster's low speed a bit.
Defensive CR: 145 HP -> base defensive CR 5. It only has two saves (only one a good save), so no adjustment there. +1 effective Defensive CR for AC 3 higher than the expected AC at CR 5. -> defensive CR 6.
Average to CR 4.

CR 3 and CR 5 variants

• CR 3 (700 XP): Decrease the icicle turtle's hit points to 101 (7d20 + 28).
• CR 5 (1,800 XP): Increase the icicle turtle's hit points to 160 (11d20 + 44). The icicle turtle's proficiency bonus becomes +3, raising its its saving throws, skills, to-hit modifiers and save DCs by 1, except for its Stealth skill, which increases by 2 instead because the turtle has expertise in that skill.

Icicle Turtle Suggestions

• Use a rollable table on Roll20 (or similar elsewhere) to easily switch the sprite between the recharges (and the "sleeping") mode.
• Make the recharge rolls at the END of the icicle turtle's turn, not the start. Same effect, but you get to plan it ahead of time, show the changes in sprite, and your players get to strategize around the coming attacks.

Ice & Cold Hazards

To accompany the icicle turtle, I also wrote some rules for some icy hazards in its lair.

• Stalagmicicle patch. Each 5x5x5 ft. cube of icicle spikes (stalagmicicles) has AC 13, 4 hit points, and is difficult terrain. Whenever a creature travels in an stalagmicicle cube (perhaps because it slammed into one after sliding on slick ice), that creature takes 1d4 (2) cold damage and 1d4 (2) piercing damage unless it succeeds on a DC 14 Dexterity (Acrobatics) check. Once a such a 5x5x5' cube has dealt damage, it crumbles and no longer causes damage and is not difficult terrain.
• Ice patch. A 5'x5' patch of ice has AC 13, vulnerability to fire damage, and hit points dependent on its thickness (see below). If its slick surface is smooth and uncovered, a creature without ice walk (or a similar ability, such as that granted by spider climb) that attempts to move on the ice falls prone and slides in their intended direction of movement along the ice until they are stopped by an obstacle; like falling, this forced movement does not provoke opportunity attacks or cost any of the creature's speed. At the DM's discretion, a creature may attempt a DC 20 Acrobatics check to move 5' on the ice without sliding; they have advantage on this check if they are prone.
  • Thick ice patches have 10 hp.
  • Thin ice patches have 3 hp. Whenever a creature that weighs at least 100 lbs. walks on a thin ice patch, that ice patch becomes cracked. The next creature to walk onto that patch destroys it and falls through to whatever is below.
• Frigid water. When a creature enters the frigid water (at least 50% immersed) for the first time on a turn or starts its turn there, it takes 1d4 cold damage and its speed is reduced by 5 ft./2 cold damage that it takes this way until the end of the turn.
  • Bitterly frigid water. As frigid water, except the creature takes 2d4 cold damage instead.

The cracks are tesselated

"cracks" is also its own separate image
if you want to use a different ice tile.
And for easy click-and-drag insertion
into a digital map mid fight.