D&D 5E Monster Conversion help? The Singing Tree

I'm trying to convert the Singing Tree from the AD&D Monstrous Compendium Annual Volume 1. A description of the original monster can be found at this blog, but basically it's a non-or-barely-intelligent tree that has a charming song it uses to lure creatures to it, where it then uses its needle-sharp leaves to drain the victim's blood (without the charmed victim's awareness), before recalling the leaves back to the tree which appear to return to its branches as if blown by the breeze.

I have a villain who has planted one in his palace gardens, training it like a guard plant and using it to test new recruits (or feed the unpromising ones to it). The massive palace gardens have a mystical Persian vibe to them and are full of dangers. Also, the tree has "overheard" (more an autonomic thing, since it's not truly intelligent) a magic password song which clever PCs might be able to "trick" it into revealing.

How does this look to you? Am I getting the CR right?

I especially struggled with determining how many extra HP to give it thanks to Needle-leaf (which is similar to a Vampire's Bite) & how to "price" its Sonambulant Song trait.

Thanks!



EDIT: Error in final formula. Should read: Total CR 13 = (16 + 10) / 2

I see a few things that, at least on first glance, do not match up/seem to "not be 5e"/might warrant a closer look. This is what I noticed after reading through things and running the numbers through my CR calculator.

1. Sonambulent Song: This strikes me as something Charisma based, which means the DC would be 11. This is a little low for A CR 13 creature, but works for something that could have dire ramifications for failing. I do not think a new saving throw should happen at the end of each round, as a creature charmed at the limit of the 100 ft. range would take 4 turns to get close enough. I would suggest that they get a new saving throw at the beginning of a turn after they take damage. Sitting down works as a description, but mechanically is not really here or there. Effects of being charmed, such as the tree having advantage on attack rolls against charmed creatures or charmed creatures automatically failing strength and Dexterity saving throws should probably be here rather than further down the sheet.

2. Needle-leaf: Reach of 5 feet might be a little short for a Huge size creature. The wording is a little wonky with regard to area effect spells (such as fireball​ ) as those do not target a creature but the area it resides in. I am visualizing the floating leaves around a 5 foot (or 10 foot) radius around the Huge tree, and it would take a perfectly placed fireball to hit all the leaves. That coupled with specifically naming fireball seems a bit out of place. Since it is already vulnerable to fire damage and will likely fail all Dexterity saving throws anyway, I would suggest either leaving that part out since the fire damage should offset the regeneration or treat it like a troll and cancel the regeneration when the tree takes any fire damage.

3. Flurry of Leaves: It took me a minute or two to figure out that the first round of damage probably came from Flurry of Leaves. If that is the case, the first round damage is too high. Calculation of area of effect damage is often done by applying the damage to two creatures that failed their saving throws. This would mean that the Flurry of Leaves does 24 damage the first round. Although this does not change things too much. Changing the damage does not move the overall CR calculation more than .5 though. It may be sufficient to say that the area within 10 feet of the tree is heavily obscured. I would also suggest that rather than doing damage immediately, the damage happens when the creature first enters the area or starts its turn in the area. I would score the disadvantage inducing part of this like Frightful Presence/Horrifying Visage. If you are using it against level 10 PCs it gets a boost of 25% of its base hp. Otherwise ignore an hp boost.

4. Regeneration: Other creatures that have regeneration do it whenever they have received damage (barring any limitations like taking fire damage) which allows it to be applied on all 3 rounds. The tree regenerates based on damage it does, and the regeneration does not take effect until the following turn. Because the tree seems to start with Flurry of leaves, it does not receive healing on the first round or the second. This leaves (no pun intended) the attack on the second round healing it on the third round. So only 60 hp of healing would get factored in as only the first 3 rounds are looked at for CR calculations.

Overall:
HP: 138 HP + 34.5 (2 resistances) + 34.5 (Flurry of Leaves/Horrifying Visage) + 60 (Regeneration on received on round 3) = 267
AC: 14 + 2 (Magic Resistance)
DCR: starts at 14 and drops to 13
DPR: 48 (Flurry of leaves and 2 rounds of 8 Needle-leaves)
AB: 9
OCR: starts at 7 and increases to 8

As it stands, from my interpretation of things, the CR is 10.5.
CR: 10.5

Thanks [MENTION=6776887]Tormyr[/MENTION], great analysis! Leaf off the puns

This is for a roughly 15th-16th level party, so no Flurry of Leaves (similar to Horrifying Visage) defensive benefit. However, I'm thinking I should treat the "advantage to attack creatures charmed by tree" similar to Pack Tactics. Here are my revised CR calculations...

Defense CR 11
eff. HP 232.5 = 138 HP * 1.25 (for resistances) + 60 HP (regained from needle-leaf)

eff. AC 16 = AC 14 + 2 for Magic Resistance

CR 11

Offense CR 9
DPR = 48

Round 1: Flurry of Leaves 12.5 * 2 PCs = 25
Round 2: Multiattack 7.5 * 8 = 60
Round 3: Multiattack 7.5 * 8 = 60
(25+60+60) / 3 = 48.33

eff. Attack +10 = 9 + 1 (conditional advantage from Somnambulant Song, similar to Pack Tactics)

CR 7, bumped up to 9 due to attack bonus

Total CR 10 = (11+9) / 2

...

I'm thinking of combining the Singing Tree with some terrain hazards, or minor monsters, or something to make it a little more interesting for a high-level party.