Example of converted Pathfinder monster - as here, the CR generally works out 2-3 higher:I'm currently running Shattered Star - Dead Heart of Xin converted from Pathfinder, see Varisia: Curse of the Crimson Throne, Runelords of the Shattered Star
I convert a lot of the Pathfinder monsters myself, as well as using Tome of Beasts and Sandy Peterson's 5e Cthulu Mythos book, which has some ridiculously deadly stuff like the Flying Polyps, never mind actual Great Old Ones! Primeval Thule 5e also has some good stats for Mythos critters. But the main thing for me is converting over the PF stat block to 5e, with a few tricks like 5e AC and other DCs = (0.5 x PF AC/DC) + 5, and halving stats over 20. It's generally much quicker to convert a PF stat block to 5e than to read & understand it fully, since I ignore most PF Feats and other heavy crunch elements. I cap DCs at 30, keeping most to around 25, and I cap to-hit bonuses at +20 with most kept to around +15, or for monsters whatever derives from Proficiency Bonus + Attack Bonus. I tend to increase hp +50%, but anything from keeping as-is to +100% can work depending on the monster. Damage is generally increased +50%, except for spell damage.
5e Monsters - Rune Giant
This giant’s skin is black and pitted, like roughly cast iron, and etched with glowing red runes.
Rune Giant CR 20
LE Gargantuan humanoid (giant)
Init +0; Senses low-light vision; Passive Perception 21
Str 30 (+10), Dex 11 (+0), Con 24 (+7), Int 14 (+2), Wis 21 (+5), Cha 18 (+4)
Good Saves STR +16 CON +13 INT +8 WIS +11
Immune cold, electricity, fire
Speed 50 ft.
Melee Greatsword (2 attacks) +16 dam 45 slashing (crit 67 slasing on 19–20) and an SML target is Pushed back 10' and knocked prone.
Ranged Javelin +16 dam 31 piercing (crit 46 piercing/19-20) plus 21 lightning damage; range 60'/240' and a SML target is Pushed back 10' and knocked prone.
Space 20 ft.; Reach 20 ft.
Equipment: Greatsword, 2 javelins.
Special Attacks command giants, runes, spark shower
Spell-Like Abilities (Spell Save DC 18; Spell Attack +10)
At will—charm person; suggestion
3/day—mass charm monster; dominate person
1/day— true seeing; air walk
Skills Athletics +16; Craft +8; Knowledge (history) +8, Knowledge (nobility) +8, Perception +11
Languages Common, Giant, Terran
Giants have disadvantage on saves vs charm or compulsion effects used by rune giants.
Whenever a rune giant uses its spark shower or spell-like abilities, it can cause the runes on its body to flash with light. All creatures within 10 feet of the giant must make a DC 18 CON save or be Blinded until the end of their next turn.
As an action, a rune giant can cause a shower of sparks to erupt out of one of the runes on its body. These sparks function as a 30-ft. cone; 45 fire and 45 electricity damage; DC 18 DEX save half; recharge 5-6.
Legendary Resistance 3/day: The Rune Giant can choose to pass a failed save.
Legendary Action 1/turn, max 3/round: At the end of another creature's turn the Rune Giant may make a Greatsword attack or use Spark Shower (if available).