Avici in the Mahayana

Question

It is very well known that it is "terrible karma", beyond even Amida's assistance, to physically draw the blood of a Buddha. Is that any kind of Buddha, a zen or vajrayana or scholastic master, or only Sakyamuni and the next Buddhist messiah?

Answer 1

AN 5.129 uses the term Tathagata. It refers to Buddhas.

Please see "Why does the Buddha call himself the Tathāgata?".

“Mendicants, these five fatal wounds lead to a place of loss, to hell.
What five?

Murdering your mother or father or a perfected one; maliciously shedding the blood of a Realized One; and causing a schism in the Saṅgha.
Mātā jīvitā voropitā hoti, pitā jīvitā voropito hoti, arahaṁ jīvitā voropito hoti, tathāgatassa duṭṭhena cittena lohitaṁ uppāditaṁ hoti, saṅgho bhinno hoti.
AN 5.129

In "Arahants, Bodhisattvas and Buddhas" by Ven. Bodhi, we see an explanation on the difference between Buddhas and Arahants:

At SN 22:58, the Buddha says that both the Tathāgata and the arahant disciple are alike in being liberated from the five aggregates: form, feeling, perception, volitional formations, and consciousness. So, what is the difference between them? The answer the Buddha gives points to temporal priority as the distinction: the Tathāgata is the originator of the path, the producer of the path, the one who declares the path. He is the knower of the path, the discoverer of the path, the expounder of the path. His disciples dwell following the path and become possessed of it afterwards. But they both walk the same path and attain the same final goal.

“The Tathagata, bhikkhus, the Arahant, the Perfectly Enlightened One, is the originator of the path unarisen before, the producer of the path unproduced before, the declarer of the path undeclared before. He is the knower of the path, the discoverer of the path, the one skilled in the path. And his disciples now dwell following that path and become possessed of it afterwards.
SN 22.58

Answer 2

The Mahāyāna recognizes up to four classes of Buddha: Āryabodhisattva, Śrāvakabuddha, Pratyekabuddha, and Samyaksaṃbuddha. A Śrāvakabuddha is an Arhat. These are not always classified as "Buddhas" in the Mahāyāna. Their Buddhahood is only with respect to the (lack of) defilements. They lack the particular bodhicitta-stabilizing insight into śūnyatā that is associated with the Bodhisattvas, and they lack the particular knowledge of the dharmadhātu that characterizes a Samyaksaṃbuddha.

Two of the above categories, the Āryabodhisattva and the Samyaksaṃbuddha, are known as "Tathāgatas" in the Mahāyāna. See the description of the 10th bodhisattva bhūmi in either the Buddhāvataṃsakasūtra ("Flower Garland Sūtra") or the Abhisamayālaṃkāra treatise of Venerable Maitreyanātha for more.

Even the above four are not universal. With respect to the buddhadhātu, all living beings are Buddhas (for this I would direct the OP to any one of the tathāgatagarbha sūtras or the Ratnagotravibhāga). It depends upon your frame of reference.

Traditionally, shedding the blood of any one of the four listed would qualify.

In some very early Mahāyāna sūtras, this terminology is much looser. "Arhat" can mean "Samyaksaṃbuddha" in some circumstances, such as in the opening gāthā of the Threefold Lotus Sūtra (which is addressed to Gautama Buddha):

We all, the eighty-thousand throng of many, entirely, without exception, with oneness of intent and gesture, bow together in worship and homage to the Arhat who has ended proliferation, ended attachment, ended ideology, ended discourse, ended consciousness, ended thought, ended mind, and ended will; who instructs elephants; who trains horses; who is the Sage unrivalled in liberation.