I’m homebrewing a creature that has the ability to summon things that buff its attacks and gives it resistances/immunities. The stat block is shown below.
Huge monstrosity, unaligned
Armor Class 17 (Natural Armor) Hit Points 479 (29d12+290) Speed 15 ft.
STR 24(+7) DEX 10(+0) CON 30(+10)INT 3(-4) WIS 10(+0) CHA 7(-2)
Saving Throws STR +14, CON +15, WIS +7
Skills Intimidation +17, Perception +14
Damage Immunities Poison Condition Immunities Blinded, Deafened, Frightened, Poisoned Senses Darkvision 60ft., Passive Perception 14 Languages — Challenge 24 (62,000 XP)
Amphibious. The toadstool can breathe air and water.
Fungal Body. Any critical hit against the toadstool counts as a normal hit.
Buff. The toadstool becomes more powerful depending on the number of sporecaps summoned, as seen below.
1-4: The toadstool deals an extra 1d10 poison damage on all attacks, it can release an extra boomshroom.
5-7: The toadstool deals an extra 2d10 poison damage on attacks, it gains resistance to all damage from piercing, slashing, & bludgeoning damage from nonmagical attacks, and it can release an extra 2 boomshrooms.
8+: The toadstool deals an extra 3d10 poison damage on attacks, it gains immunity to all damage from piercing, slashing, & bludgeoning damage from nonmagical attacks, resistance to all damage from piercing, slashing, & bludgeoning damage from magical attacks, and it can release an extra 3 boomshrooms. The toadstool must be within 30 feet of the sporecaps in order to gain these benefits.
Multiattack. The toadstool makes three attacks, one with its bite and two other actions.
Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 6) piercing damage, and the target is swallowed if it is a Medium or smaller creature. A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the toadstool, and takes 10 (3d6) acid damage at the start of each of the toadstool’s turns. The toadstool’s gullet can hold up to two creatures at a time. If the toadstool takes 20 damage or more on a single turn from a creature inside it, the toadstool must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, each of which falls prone in a space within 10 feet of the toadstool. If the toadstool dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 10 feet of movement, exiting prone.
Sporecap.(Recharge 4-6) The toadstool can use its action to summon 1d4 sporecaps. Sporecaps are large, tree-size mushrooms. The sporecaps buff the toadstool. The number of sporecaps determines how powerful the toadstool is. These sporecaps have 40 (3d10+24) hit points each.
Boomshroom. The toadstool releases 4 (1d8) Boomshrooms. Boomshrooms are small mushrooms that grow and explode, and any creature within 5 feet of the it takes 12(2d12) poison damage.
Ground Pound. The toadstool can hop on the ground, and any creature within 25 feet of it must succeed on a DC 15 Dexterity saving throw, taking 27(3d12+7) bludgeoning damage on a failed save, or half as much on a successful one. This radius then becomes difficult terrain. The toadstool ignores the difficult terrain.
The toadstool can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The toadstool regains spent legendary actions at the start of their turn.
Spore Bomb. The toadstool attaches a spore to a creature of its choice within 20 feet of it. This spore explodes at the end of the target’s next turn, creating a 10-ft radius of necrotic gas for 1 minute. Anyone who enters the space of the cloud takes 13 (3d8) necrotic damage for each round they spend their turn inside the gas.
Ground Pound (Costs 2 actions.) The toadstool uses its ground pound.
How would the buff ability affect challenge rating?