Guildmages' Forum is a rare Land card from Guilds of Ravnica (GRN), collector number 250. Current market price: $0.39 (foil $1.24), TCGplayer market prices via Scryfall.
{T}: Add {C}. {1}, {T}: Add one mana of any color. If that mana is spent on a multicolored creature spell, that creature enters with an additional +1/+1 counter on it.