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I have used CR calculators to get an idea of the difficulty of this encounter, but I thought the collective knowledge here may provide me with a better answer than I could find myself, as the encounter is somewhat unorthodox and I have no DM experience.

I calculated the tyrannosaurus at CR6 and Akena at CR1, but have difficulty with it because CR doesn't seem to look at anything other than HP, AC, Average DPR, and Attack Bonus. Also, I'm unsure as to how much an encounter should be scaled up when CR assumes 4 PCs and I have 5-6.

The first big milestone in the campaign involves a creature somewhat of my own design, based off a tyrannosaurus and a sahuagin priestess, the dinosaur being essentially a mount for the priestess. The creatures are as follows:

Akena's statblock:

CR 1
45 HP
AC 15 (with mage armor) Perception +6,
Passive perception 16

Blood Frenzy: Advantage on attack rolls against creatures with less than their max hp.

Akena has the Mounted Combatant feat.

STR 8 (-1)
DEX 14 (+2)
CON 10 (0)
INT 14 (+2)
WIS 10 (0)
CHA 16 (+3)

Spellcasting Ability - Akena is a 6th level spellcaster with spell save DC14, +4 to hit with spell attacks. She has the following spells prepared:
Cantrips - Poison Spray, Ray of Frost, Shocking Grasp
1st level (4 slots) - Mage Armour, Magic Missile, Witch Bolt
2nd level (3 slots) - Snilloc's Snowball Storm, Enhance Ability
3rd level (3 slots) - Tidal Wave, Wall of Water

Tyrannosaurus (MM page 80) statblock:

CR 6
AC 13
136 HP
Speed 50ft
Perception +4
Passive perception 14

STR 25 (+7)
DEX 10 (0)
CON 19 (+4)
INT 2 (-4)
WIS 12 (+1)
CHA 9 (-1)

Actions:
Multiattack: Makes two attacks: One with its bite and one with its tail.
Bite: (+8 to hit) 3d12+7 (reduced from 4d12+7) damage on hit. Target is grappled (DC 17 save.) Until this grapple ends, Tyrannosaurus can't bite another target.
Tail: Melee weapon - (+8 to hit) reach 10ft, 2d8+7(reduced from 3d8+7) bludgeoning damage.

The party consists of a Lizardfolk Barbarian, a Tortle Ranger, a Wood Elf Rogue, a Sea Elf Sorceror, and a Triton Druid (all level 5). They will be accompanied by a ranger NPC I've created that is slightly less powerful than them. The Barbarian and Ranger PCs are experienced players.

Is this encounter too difficult/easy?

This is meant to be the moment in the first third of the campaign where the main plot hook is partially revealed to the players, and I want the encounter to be difficult and somewhat nerve-wracking, ideally knocking 1-2 PCs unconscious but I certainly don't want a TPK.

The main thing giving me trouble considering the difficulty of the encounter is Akena, being mounted atop the tyrannosaurus and being able to spellsling in relative safety. I forgot to add the Mounted Combatant feat to her when originally posting this question. I've nerfed the damage of the Tyrannosaurus a bit to compensate. Should I have left the damage as is in the Monster Manual?

If I take the suggestion in the answer below and add 2 CR2 Allosaurus, That would take the CR of the encounter up to CR11. Is that something I should regularly expect my PCs to be able to defeat?

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closed as unclear what you're asking by Miniman, daze413, Ruse, Sdjz, Szega Oct 11 '18 at 8:56

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ For us to be able to help you, you need to define "too easy" and "too hard". \$\endgroup\$ – Miniman Oct 11 '18 at 7:06
  • \$\begingroup\$ What level are the PCs? Have you figured out the Challenge Rating of Akena? If you show us your efforts in trying to solve your problem, we can improve upon what you started and help break down the block you're having, and makes for a much much better answer. \$\endgroup\$ – daze413 Oct 11 '18 at 7:24
  • \$\begingroup\$ @daze413 I'd say Akena is CR2? First I used the general rule for Player like NPCs but then I reread the question and they mention "sahuagin priestess" which is CR 2 according to a quick google search. \$\endgroup\$ – JDM7 Oct 11 '18 at 8:27
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    \$\begingroup\$ @ZynMarkov it looks like you've accidentally created 2 accounts (which tends to happen when a user registers their account after posting a question as an unregistered user). Please follow this link for information on how to merge them so that you can freely edit and gain rep for this (and future) questions :). \$\endgroup\$ – Purple Monkey Oct 12 '18 at 0:37
  • \$\begingroup\$ Thanks for clarifying the CR of both creatures, we still need the levels of the PCs, though. Also, do you have the DMG? There's a section in the DMG that teaches you how to estimate the difficulty of an encounter given the number of creatures, their CRs, and the number of PCs and levels. Have you taken a look at that? \$\endgroup\$ – daze413 Oct 12 '18 at 7:51
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This encounter should be relatively easy.

The fact that Akena is mounted on top of the T-Rex doesn't provide her with much protection RAW unless you plan on giving her some form of written bonus (Mounted Combatant feat or similar) given they will still be in range of the Sorceror, Druid, 2 Rangers and possibly the Rogue - combine this with her squishy AC and HP and I think it's unlikely that she'll last longer than 1 round if they focus her down.

That then leaves a party of 5 level 5 PCs (and a helper NPC) against a single CR6 creature. The way 5e combat math works (due to bounded accuracy) greatly favours the side with more creatures. The barbarian will probably be able to tank the T-Rex for several rounds (especially if healed by the Druid) while the rest of the part take the T-Rex down from long range. Bear in mind that the T-Rex has terrible INT/WIS/CHA saves, so if the Sorceror has certain single-target disabling spells at their disposal then they could trivialise the fight entirely.

I'd recommend adding a couple of smaller creatures (CR2 or so) to even out the numbers a bit, as well as providing Akena with some more protection.

I say should since it depends on somewhat clever play from your PCs, which is by no means guaranteed :)

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