When Harold and Bob dies, if it was a creature, return it to the battlefield. It's an Aura enchantment with enchant Forest you control and "Enchanted Forest has ': Add three mana of any one color. You get two rad counters.'" Harold and Bob loses all other abilities.
Cast Harold and Bob early as a 3-mana vigilance/reach body, then intentionally kill it so it returns as an Aura that turns one of your Forests into a 'tap for three mana of any color' rock—giving mono-green decks both explosive ramp and full color fixing. Use that surplus mana to power out enormous threats, X-spells, or mana-hungry combo pieces while the rad counters quietly fill your graveyard for recursion payoffs.
Converts into permanent ramp AND off-color fixing, the biggest weakness of mono-green decks
Cheap to deploy and resilient—removal that kills the creature just upgrades it into the enchantment
Three mana of any one color from a single Forest tap enables huge mana spikes and splashy effects
Vigilance and reach make it a fine attacker and defender before you convert it
Needs a sacrifice outlet or combat death to flip into its valuable Aura form
Rad counters mill you and cost life on nonland cards, which can be a real clock against burn or mill-light grindy boards
The Aura is vulnerable to enchantment removal and the Forest it's attached to dying
Mono-green color identity limits interaction and answers to fast combo
Classic mono-green payoff that dumps your deck onto the battlefield off the explosive mana.
Add the cheapest and free sacrifice outlets (Ashnod's Altar, Phyrexian Altar, Birthing Pod) so you can convert Harold on your terms the turn it lands. Stack untappers and mana doublers—Seedborn Muse, Vernal Bloom, Vorinclex, Nyxbloom Ancient—to turn one Forest tap into a windmill of any-color mana, then add a deterministic mana sink like Walking Ballista or Finale of Devastation to close. Tutors like Green Sun's Zenith and Worldly Tutor improve consistency, and proliferate or graveyard payoffs let you turn the rad-counter downside into card advantage.
Legendary Creature — Treefolk Mutant
: Add three mana of any one color. You get two rad counters.'" Harold and Bob loses all other abilities.