2021-06-18 : If a planeswalker's loyalty ability normally has a cost of [+1], Carth's ability makes it cost [+2] instead. A cost of [0] would become [+1], and a cost of [-6] would become [-5].

2021-06-18 : If an effect replaces the number of counters that would be placed on a planeswalker, such as that of Vorinclex, Monstrous Raider, that replacement happens only once, at the time payment is made.

2021-06-18 : If you somehow manage to control two Carths (perhaps because of Spark Double), the cost-changing effect is cumulative. In total, loyalty abilities will cost an additional [+2] to activate.

2021-06-18 : The total cost of a planeswalker's loyalty ability is calculated before any counters are added or removed. If a loyalty ability normally costs [-3] to activate, you do not remove three counters from it and then put one counter on it. You remove two counters at one time when you pay the cost.