Martyr's Soul

Text:

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for or one mana of that creature's color.)
When Martyr's Soul enters the battlefield, if you control no tapped lands, put two +1/+1 counters on it.

Rulings:

• 6/14/2019 If you control any tapped lands as Martyr’s Soul enters the battlefield, its last ability doesn’t trigger at all. If you control any tapped lands as the ability resolves, you won’t put two +1/+1 counters on Martyr’s Soul.
• 6/14/2019 You can tap an untapped creature you haven’t controlled continuously since the beginning of your most recent turn to convoke a spell.
• 6/14/2019 When calculating a spell’s total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn’t change a spell’s mana cost or converted mana cost.
• 6/14/2019 If a creature you control has a mana ability with <img src="/Handlers/Image.ashx?size=small&amp;name=tap&amp;type=symbol" alt="Tap" align="absbottom" /> in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell’s costs. You won’t be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won’t be on the battlefield when you pay the spell’s costs, so you won’t be able to tap it for convoke.