Thallid is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

At the beginning of your upkeep, put a spore counter on this creature. Remove three spore counters from this creature: Create a 1/1 green Saproling creature token.

How it works : Thallid

Original English text — Wizards of the Coast / Scryfall

No additional ruling has been published for this card.

Popular combos

Real combos played by the community, sorted by popularity.

#1
Evolution Sage + Life and Limb + Sporoloth Ancient + Thallid
Infinite creature tokensInfinite creature ETBInfinite landfall triggersInfinite proliferate
1Activate either Thallid or Sporoloth Ancient by removing two spore counters from it, creating a 1/1 Saproling creature token.
2When the Saproling enters the battlefield as a Forest land due to Life and Limb, Evolution Sage triggers, putting a spore counter on Thallid and Sporoloth Ancient.
3Repeat.

#1
Evolution Sage + Life and Limb + Sporoloth Ancient + Thallid
Infinite creature tokensInfinite creature ETBInfinite landfall triggersInfinite proliferate
1Activate either Thallid or Sporoloth Ancient by removing two spore counters from it, creating a 1/1 Saproling creature token.
2When the Saproling enters the battlefield as a Forest land due to Life and Limb, Evolution Sage triggers, putting a spore counter on Thallid and Sporoloth Ancient.
3Repeat.

#2
Vorel of the Hull Clade + Intruder Alarm + Thallid
Infinite creature tokensInfinite creature ETBInfinite mana creatures you control can produceInfinite proliferate for artifacts, creatures, and lands
1Activate all mana-producing creatures you control by tapping them, adding at least .
2Activate Vorel of the Hull Clade by paying and tapping it, doubling the number of counters on Thallid.
3Activate Thallid by removing three spore counters from it, creating a 1/1 Saproling creature token.
4When the Saproling enters the battlefield, Intruder Alarm triggers, untapping all creatures.
5Repeat.
6If creatures you control can tap to produce at least , this combo nets infinite mana and you may target any creature using Vorel of the Hull Clade.
7This combo also nets infinite spore counters on Thallid if it has four or more spore counters to start the combo.

#2
Vorel of the Hull Clade + Intruder Alarm + Thallid
Infinite creature tokensInfinite creature ETBInfinite mana creatures you control can produceInfinite proliferate for artifacts, creatures, and lands
1Activate all mana-producing creatures you control by tapping them, adding at least .
2Activate Vorel of the Hull Clade by paying and tapping it, doubling the number of counters on Thallid.
3Activate Thallid by removing three spore counters from it, creating a 1/1 Saproling creature token.
4When the Saproling enters the battlefield, Intruder Alarm triggers, untapping all creatures.
5Repeat.
6If creatures you control can tap to produce at least , this combo nets infinite mana and you may target any creature using Vorel of the Hull Clade.
7This combo also nets infinite spore counters on Thallid if it has four or more spore counters to start the combo.

Interactions with other cards

These cards' text mentions Thallid.

Blightreaper Thallid // Blightsower Thallid

{3}{G/P}: Transform this creature. Activate only as a sorcery. ({G/P} can be paid with either {G} or 2 life.) When this creature transforms into Blightsower Thallid or dies, create a 1/1 green Phyrexian Saproling creature token.